Relative light loss of 35mm adapters' question

D

Daniel Wee

Active member
Hi,

I'm agonizing over my decision of which 35mm adapter to get and one of the questions I can't seem to get around is the light loss issue. I wonder if anyone here who has had experience with more than one 35mm adapter would be able to share and shed some light (no pun intended) on the relative light loss of the models they have tried.

Right now from what I can surmise, it would appear as if the Letus 35 Flip Enhanced has the biggest light loss due to the Flip system. What I don't know is by how much, since reports have stated up to 4-stops of light loss in not so well lit situations. A possibly unrelated issue is what appears to be a color cast with this adapter. The site states a 1.5-2 stop loss without the lense and with the lense I presume this will increase. Most of the clips shot using the Letus appears to be somewhat on the dark side of things.

The Brevis, on the other hand, and the other end of the scale it would seem, actually boasts a 0.5-stop GAIN.

The other two I'm looking at are the iFocus35 and the Go35Pro but I can't quite place them on this scale as the information available seems to be more anectdotal than absolute. I'd be interested, too, to see where the M2 stands alongside these adapters in terms of light loss.

What would be really great would be screen grabs of an indoor scene without adapters, and then with (each) adapter mounted, and with the camera settings completely on manual. This way we can see visually what we're dealing with here. I know it's a lot to ask so I'm not really expecting this but any light you can shed on this issue will be very much appreciated.

Thanks.

Daniel Wee

Letus 35FE
Pros: Readily available, upright image
Cons: Light loss, color cast, thread alignment

Brevis35
Pros: Low light loss, small & light, High quality custom achromat
Cons: More expensive than Letus, iFocus, or Go35, Unknown bokeh

Go35Pro
Pros: Small, cheaper than most adapters, nice bokeh, has high quality achromat, short lead time
Cons: Static diffuser
Issue: Not sure how light loss compares to Brevis and or Letus

iFocus35
Pros: Actually very similar to Go35Pro but not sure about specific performance
Cons: Static diffuser, more expensive than Go35Pro, longer lead time than for Go35Pro
Issue: Not sure how much better performance the money gets you over Go35Pro

Other potential contenders - Digi35
 
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i will try to do something with the M2 this week, and ill post back here. but remember, loss of light isnt a big deal. Mounting one of these things isnt for just running around. When outdoors, during the day, you will have no probs with lighting, when in doors, you will have wanted to use pro lighting anyways, and at night, well you will want to have used pro lights again, so the loss of light doesnt really matter in the end.

If you have a 35mm adapter, you should have some lights. Usually the lights come first.
 
I can't speak for the other products out there, but the Brevis unit does indeed show up to .5 f stops gain, but this is only adapter loss. Add in the light loss from the 35mm lens to calculate expected total system loss. In other words:

Brevis adatper = .5 stops gain
50mm f1.4 lens with fully open = 1 stop loss

Total system loss = .5 stops loss

What this means is that with decently sensitive cams like the DVX, most people don't really notice the light loss on fully opened lenses. The comments on adding light are very relevant though as for best image quality, you shouldn't really be using 35mm lenses with fully open apertures. Once you stop down your 35mm lens, light loss becomes a big factor in terms of how much light needs to be added indoors. 1 stop means double the light!
 
I recently built a homemade static 35mm adaptor based on Edweirdo's design. Although it works fairly well in the day outside, you need to fully open the apeture on your 35mm lens. This results in more light coming in, but gives you a much shallower depth of field. If you stop down the apeture, you will get a deeper depth of field, but it will be too dark and you will see your focusing screen.

Snodart is in the process of putting together a DIY manual and DVD with step by step instructions on how to build a vibrating adaptor:

http://www.dvxuser.com/V6/showthread.php?t=54639

Check out his pics/video on page 8 of this thread. I think they're quite impressive, especially considering that he shot the footage at sunset! If I were you, I'd build his design. It's cheaper than buying one of the other adaptors out there. Plus you'll have the satisfaction of knowing that you can build it yourself (assuming of course you have access to the right tools).

Mike
 
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Hi Dennis,

Thanks for the clarification. A related question I have is concerning the impact of the design. With the Brevis diffuser being so transmissive in it's nature, is there any impact on the amount of diffusion and hence image quality, such as bokeh? Also, does the new achromat introduce any further loss? Thanks.

Daniel
 
Daniel, bokeh quality and rendering is a very subjective topic, and one person's great bokeh, is not necessarily another's. Furthermore, 35mm lenses vary a great deal through the aperture range, and between manufacturers, in how they render bokeh. We've focused on what's important...matching 35mm film bokeh as closely as possible. With that said, the Brevis now has two diffusers which are designed to provide a match from f1.4 to f16. The "stock" diffuser is a very good match from f2.8 to f16 and offers the best mix of transmission, diffusion, sharpness, and grain (no grain).
 
guskersthecat said:
I can't speak for the other products out there, but the Brevis unit does indeed show up to .5 f stops gain, but this is only adapter loss. Add in the light loss from the 35mm lens to calculate expected total system loss. In other words:

Breves adapter = .5 stops gain
Click to expand...

This statement really struck me as illogical.

I would be extremely interested to learn your testing criteria and the conditions necessary for any passive optical device to exhibit "gain"!
 
It makes more sense when you consider that you are not simply using the 35mm lens adapted directly to the video cam's lens. You are capturing a projected image. Scaling factors are different, as are the optics of the adapter itself. With fast lenses, it is also possible to use a frame size on the Brevis larger than the 24x36 standard.

The best validation of this is not to take my word for it (I'm no physicist), but rather note what current users are reporting in their posts :-)
 
guskersthecat said:
It makes more sense when you consider that you are not simply using the 35mm lens adapted directly to the video cam's lens. You are capturing a projected image. Scaling factors are different, as are the optics of the adapter itself. With fast lenses, it is also possible to use a frame size on the Brevis larger than the Cox standard.

The best validation of this is not to take my word for it (I'm no physicist), but rather note what current users are reporting in their posts :-)
Click to expand...

My background is physics, so I have a great deal of difficulty with this statement and even more so with concept of using hearsay as a proof.

My simple question remains, can you state your testing criteria that results in the claim of "gain"?
 
The reason I had not included the SGPro really is because the website has been down for so long, I'm not sure it's still in contention. Actually the Brevis is already just out of range here at $995. Remember with these non flip units, you may have to invest in additional monitors which will add to your overhead. For those with the DVX100/100A there is the option of using the magnet flip but for those using the DVX100B/102B, you have to either do a hack on the LCD hinge switch which can void the warranty, or resort to the external monitor, external battery to power the monitor, monitor mounting bracket, etc. This is one reason why the Letus35FE is still so attractive in spite of the high light loss. Imagine running and gunning with all that stuff hanging off your rig.

The Go35Pro ($650) is sufficiently cheap to at least allow for the purchase of an external monitor and still stay below the $1k margin. However, it is a static solution.

Herein lies my dilemma:-

1. Letus 35FE, cheap, convenient, no external monitor required, no additional flipping in post, but requires additional lighting indoors. (readily available)

2. Go35Pro, cheap but will have to fork out for monitor, small and light, good bokeh, not sure about how the grain will be in actual usage, not sure how much light loss relative to say the Brevis. Final cost will be close to $1k after shipping, and accessories. (5-7 days processing time)

3. Brevis, expensive, even more so when monitor is added, for DVX, achromat is required so it's $995 before shipping. Advantage in low light loss and moving diffuser. But way expensive as a solution. (seems to be readily available but not sure how much processing time)

4. The iFocus35 is like the Go35 I think but not much info as yet. Long lead time (about a month from placing order to receiving it here in Asia)

5. SGPro is about $700 (400 pounds) and includes rails in the price but I don't know about availability. I probably should check with them.

At this point, the Letus35FE is probably the most convenient and attractive solution, price wise, but not quality wise due to light loss and issues with threads alignment.

The Go35Pro is probably the best (unless iFocus35 or Digi35 is competitive) both in price and quality for non-flip static solution.

The Brevis seems to have a light advantage but that may well be due to the fact that it can use a larger area than 24mm by 36mm. If the Go35 or other adapters can do this, it can theoretically have lower light loss. This is where I'm looking for information on the relative light loss to see what those figures all mean in real life.

Daniel Wee
 
Just to put things into perspective here - a number of adapters are stating around 2-stops of light loss. I've created a series of photos here that simulates the light loss from 0 to 2-stops:-

Normal - 0-stop of loss
Test-00stop.jpg


0.5-stop of light loss
Test-05stop.jpg


1.0-stop of light loss
Test-10stop.jpg


1.5-stops of light loss
Test-15stop.jpg


2.0-stops of light loss
Test-20stop.jpg


So you can see that 2-stops is actually a very severe amount of light loss. Something that is perfectly acceptable under normal shooting could be impossible or at least very difficult without extra lighting, which in some cases may not be possible. I would think that up to 1-stop of light loss is still acceptable but beyond that, it will change the way you shoot significantly, especially for run-n-gun documentaries.

Daniel Wee
 
guskersthecat said:
I can't speak for the other products out there, but the Brevis unit does indeed show up to .5 f stops gain, but this is only adapter loss. Add in the light loss from the 35mm lens to calculate expected total system loss. In other words:

Breves adapter = .5 stops gain
Click to expand...



khmuse said:
This statement really struck me as illogical.

I would be extremely interested to learn your testing criteria and the conditions necessary for any passive optical device to exhibit "gain"!
Click to expand...

I would have to agree that a gain of .5 does not make logical sense and I too would be interested in how you tested the light loss. It seems physically impossible to be able to increase the intensity or amount of light via the GG alone. Now, if the light rays were condensed (ie: plano-convex lens) then I could see how the intensity could increase slightly. But this would mean that the resulting image that is projected onto the ground glass would have to be much smaller in relation to not using a condenser lens (considerably smaller to make up for some of the light loss already occurring via the 35mm lens). I would be curious to know how powerful the condenser you are using is (assuming you use one) and which direction the convex side is facing. Not that I don't believe that your adapter has a small amount of light loss, but something seems to be amiss with a .5 gain.

In my experiences and tests while building adapters, a small amount of light loss typically meant that the focusing element was not diffusing the light enough. In these cases, with small aperture sizes on the 35mm lens (ie: f/11, f/16, f/22, etc) bright colors and areas of white tended to loose any contrast and eventually blow out (too soon). The very fine grit GG's caused nasty hot spots at these smaller aperture sizes (to match the shape of the aperture). And this is not to say that the Brevis has similar problems. It has just been my experience and I honestly haven't seen much of any footage from adapters at these higher f/stops.

Mike, thanks for the mention by the way.
 
That's and interesting link but are you counting the stops off the camera reading? If so, then it may be possible that it's not as accurate. I am wondering what kind of numbers other adapters would yield with a similar procedure.

For me, the easiest way would simply be to evaluate the system loss as a whole using a standardized lense and procedure. A light meter would be a great way to go. Part of the problem may be due to users usually reporting system loss rather than just adapter loss. So someone may be saying that they're seeing 2-stops of loss with the adapter (meaning just 1-stop loss for the adapter), and that would be the total system loss, including the lens. The Letus35FE claims a 1.5-stop loss just on the adapter alone.

Daniel
 
Clearly any claim which indicates anything other than a loss of some quantity (no matter how small) is based upon a flawed methodology. If this were not true, then an ever increasing number of adapters coupled one after another would eventually result in a nearly infinite amount of light exiting the final device. One could go further and state that if these were assembled into a chain, then a free energy system would result.

This more than bends a few fundamental rules.

I am certain that if the methodology were presented, the error would be easy to identify.
 
The methodology is provided, a few posts back. There are two components to consider. The lens, and the adapter. Both combined, I would refer to as total system loss. For a 50mm lens at f1.4 (which is what I'd propose as a standard) you would see a total system loss of .5 stops. This makes the most sense to the most people. Adapter loss itself is a measurement of questionable value, and I'd agree that using a standard lens is the best measure. In fact, we shouldn't be using f stops at all but cinema lenses calibrated in T stops. The adapter gain in my calculation is the only way to explain how an f1.4 lens, which loses one full stop, only requires .5 stops more light to the camera.

Using the camera as a measurement makes the most sense as this is what everyone is lighting for, and the most relevant question answered is simply then, "What is my camera going to see in terms of light loss"?

Remember that you cannot apply basic lens physics here because the optical chain is being broken by the GG, and furthermore, you are measuring the camera's response to the light. I'm not suggesting that the adapter is a fusion reactor capable of satisfying the world's energy needs (that would be nice though). In fact, what I am stating, is the loss that the camera will see. The method I use allows me to compare GG's on precisely the same camera, same scene, same lighing, same 50mm lens at f1.4, and same adapter. I used the identical technique during extensive testing of spinner designs, and many GG's. The best I could do with a non flip spinner (with good diffusion) was 1.5 to 2 stops of total system loss. If anyone can come up with a real world test that has fewer variables, I'm all ears.

And again (finally), for those questioning, I'll defer to the current Brevis users for real world observations...the most important ones.
 
You may also want to consider that the camera's F-stop calibration may not be accurate enough. You can verify this by stopping down the lense and see if you get an accuracy correpondence in the F-stop change on the camera. I suspect it won't correspond, meaning that the F-stop reading on the camera is inaccurate. This is also probably where a light meter would be much more precise.

Daniel Wee
 
The t stop system is in fact a much better measure system as it takes into account (normalizes) the actual light loss and not just the relationship of the diameter to the focal length.

Obviously, the optical path is effected by the use of a focus screen. In fact it is now two paths. One from the light source, through the capturing lens to the focus screen. The second path is from the focus screen through the camera's lens to it's imagers.

So for the question of how one might better quantify "the system". I would suggest that the use of a f-stop loss is a flawed approach as it is not quantifying the adapter, but rather the capturing lens/adapter combination.

Here is a potentially better approach:

Compare the performance of a calibrated (read cinema) lens via the adapter to the that of the camera alone. This would be done at the same aperture / focal length ratio to normalize the process.

If the camera is set to say f 2.8, then the cinema lens would also be set to t 2.8. Using an evenly illuminated flat light source with a neutral color balance, establish the amplitude of the image at the camera (as an example, say you measure 50% with this setup). Now, insert the capture lens/adapter combination in the path and make the same camera based measurement.

In this theoretical test example, say you now read this same illuminated source as a 25% reference. The energy differential (not taking into account the non-linear response curve of the camera) establishes the adapters loss. In this example, you would find the following:

Linear change is Power A / Power B

or

25% / 50% = .5

expressed in "f stops" .5 would be

2^(delta) where delta is .5

or

2^(.5) = t 1.414 (or a 1 stop loss for the adapter)

This would quantify the adapter's performance separately from the system (capturing lens/adapter). This methodology allows for quantification of the adapters losses. And there will ALWAYS BE A LOSS.

Your comments are welcome.
 
Kh, that sounds like a nicely sorted empirical solution. Now you just have to convince every adapter manufacturer to use the same technique :-) I believe your technique would work well in an adapter "shoot out" so that the same testers/technique would apply across the board.

I also agree, particulary after Dan D. posted up the variety of rear aperture specs on SLR lenses, that t stops offer a much better reference. Having said all that, based on my tests with the GS400, DVX, XL2, XL1s, VX2100, and HVX, users should expect to see approx. .5 f stops loss at the camera, using the Brevis (standard diffuser), with a 50mm lens at f1.4 :Drogar-BigGrin(DBG)

I'm getting the PL mount sorted out right now, so shortly I will be testing some cinema lenses. The brevis is going to be very, very happy with those gaping rear apertures!
 
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