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    Brevis/Letus SGPro comparison
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    Here are the results of my tests comparing the Letus, Brevis and SGPro Rev2 on an HVX. My preliminary findings are no clear winner. All have pros and cons. This may be even more true once they all come out with their flips in a month or so. (BTW my understanding is that the optics of the SGPro Rev2 and Rev3 are the same, but if they aren't then Rev3 mileage might vary.)

    I forgot to bring a light meter the day we tested light loss so these numbers should be considered approximate as we judged light loss on a waveform looking around the 70IRE area where caucasian faces sit. We figured about 20 IRE/stop. However the comparisons should be accurate. We used HDNorm gamma and set the knee at "high" to minimise its affect.

    We have no frame grabs sorry. Too much work. This was bad enough.

    1. Speed.

    First we tested how much the Naked HVX to see how much it dropped off with different zoom ranges. This was important because while our SGPro and Brevis both cleared vignetting @ N72, our Letus was more offset and we needed to get up to N77 for a clean shot.
    With HVX wide open b/n N60 and N80 you lose about 1/2 stop. From N72 to N77 it seemed maybe about 1/6. At f2.8 the loss was negligeable.

    Low light performance:
    The fastest of the adapters is still the Brevis with a CF1 or CF1L. My tests were only with a CF1L. It seemed about 1/2 stop slower than the Naked HVX with a Nikon 85mm @ f1.4. However, it was also about a 1/2 stop slower than the Naked HVX @ f2.8.

    Oops was that a typo? No, in fact the Brevis at least on an HVX with Nikon lenses and a CF1L does not respond to the change in iris from f2.8 to f1.4. It doesn't respond at all (actually maybe just a hair) - that means you don't gain any exposure and there is no change in DOF.

    This is a very important wrinkle in comparing adapters. I don't really know why this occurs but my own completely unscientific theory is that because the Brevis CF1L does not scatter light a great deal that the increase in the f stop doesn't have any light to gather from the sides of the lens. Its just a guess though.

    In fact this is true to a lesser extent with most of these adapters. Generally the light decrease from f1.4 to f2.8 is less than a stop, sometimes as little as a quarter stop and each f-stop as we close down reduces comparatively more exposure with each stop. My observation is that the denser the diffusing screen, the more we will see a difference between 2.8 and 1.4 and vice versa. With the Letus there was a little over 1/2 stop between f1.4 and f2.8. With the SGPro I saw a bit more mabe 2/3 stop or more. As I recall the Redrock and the Brevis CF3 may have shown even more difference

    Thus the faster the adapter is, the less opening up beyond 2.8 helps you out in low light. That's the down side. The upside is if you already have a fast adapter like the Brevis, and you want to shoot at 2.8 because the lens performs better at 2.8, then you will have an even greater sensitivity advantage over a slower more diffuse adapter that loses light at 2.8.

    Ultimately then we need two different ratings for speed comparisons. How do they perform at 1.4 and how do they perform at 2.8. Keep in mind that each combination of diffusing screen, optics, video camera and perhaps even 35mm lens may perform differently. I only looked at 3 adapters on my HVX with one Nikkor Lens. However I have seen similar results with other lenses. This is not peculiar to the 85mm

    So here are the results with an 85mm/f1.4 Nikkor on my HVX.

    @ f1.4 Brevis lost about .5 stop
    SGPro .6 stop
    Letus 1 stop (corrected to N72 maybe .85)

    f2.8 Brevis .5 stop
    SGPro 1.2 stops
    Letus 1.5 stops


    To add to this I have noticed that my Tokina 80-200 is at least 1/2 stop slower than my f2.8 Nikon primes. (at least it is with my adapters). Because the Brevis is still only slowly reacting to f stop changes as it closes iris, I would expect that with with my Tokina the difference in speed between the Brevis and SG Pro or Letus might even increase a bit more. That is what I have observed on informal tests in the field.

    2. DOF
    Speed is just one aspect of the adapters though. DOF seems to be inversely related to speed. As the diffusing screen becomes more dense, out of focus parts of the image seem to spread more and thus what I call the "Apparent DOF" of the adapter" changes. We would never see this in film of course. There DOF is DOF - its a mathematical relationship between lens focal length and target size. You can make a reliable chart for any 50mm lens on any 35mm target on any camera in the world.

    As we would expect, the SG Pro has the least DOF, The Letus comes in a close 2nd and the Brevis has the most. I suspect the Redrock might have even less than the SGPro but that's just speculation.

    3. Pro's & Cons to DOF

    In general we are using 35mm adapters for their low DOF, so "less is better". However DOF can get too low in many situations. In a dramatic piece it can be hard to hold focus in some shots and you may come to curse your adapter. Even a talking head interview can get rough if the subject likes to move. The first time someone rented my Redrock last year they brought it back and said the DOF was too low for them. The Brevis is probably closer to the DOF we expect in a normal 35mm feature. However feature cameraman sometimes fight to get lower DOF with long lenses and low f stops. It can be easier for us.

    4. Bokeh
    Bokeh is different than DOF. It refers to how the lens and in our case the diffusing screen affect out of focus parts of the image. This is much more subjective.

    Here I personally find the slower adapters to be prettier. The SGPro handles blown out images in a way that is more pleasing and natural to me than the other 2. That fits with the praise you hear on this forum for the SGPro's look. I don't have a Redrock to compare anymore, but many here also felt that it was very pretty - even the prettiest of all of them. However the increased diffusion of the screen sometimes caused some to complain that perhaps it halated too much around bright objects almost looking like a promist.

    The Brevis has the least halation and generally looks pretty clean but sometimes can put a kind of grainy look around very overexposed lights. In practical shooting, I've never noticed it, but on tests with a very overexposed light bulb, it was apparent. The Letus is in between the 2 once again, and perhaps has a bit more tendancy to create what some have called "smear" but might better be described as making out of focus bright objects look a bit like blobs.

    However be aware that most of the "blobby" types of overexposure only occur at f 1.4 and are partly due to the bokeh of the lens iris which is virtually a circle at that point. As soon as you close down it picks up the hexagonal or octagonal pattern of the diaphram which is generally prettier.

    I found the SGPro's bokeh in general the prettiest of the three though they all looked good and the flaws I've mentioned seem very minor. Personally I might want to have one faster with higher DOF and one a bit slower with lower DOF and best bokeh.

    So - what kind of look do you want. Clean or slightly diffuse. Halated lights or not. More or less DOF

    4. Sharpness - I just don't feel very comfortable comparing sharpness yet. I didn't concentrate on this in the time we had. They all look good to me and frankly every time I look at this they seem a little different. There may be an advantage to one or the other but I don't want to make half baked conclusions

    5. Zooming In
    The Letus allows you zoom in on the image to a limited extent with no appreciable loss of sharpness. This is way cool as it gives you the ability to somewhat vary your focal length without changing lenses.
    My guess is that the others may gain this ability with the flip optics.

    6. Collimation
    Both the SG Pro and and the Brevis have reliable methods for adjusting lens collimation.
    The Letus seems to have skipped this detail. I'm sure Hien & company will quickly come up with some kind of fix for that, but its a pain to set 35mm lens back focus right now and I wouldn't want to have to switch between different lens mounts in the field.
    At this point you have to hold the lens in your hand trying to make sure you don't tilt it off axis, keeping it in the right position, while you look at a monitor and and adjust 3 allen screws. Its wanky and the more the allen screws have worn into the mount the harder this may get. Actually just providing thin circular shims (like professional cameras use) could help this enormously.

    7. Build
    The Letus seems very well built but heavy. I had a problem on my HVX because it didn't have a correction for the offset on my HVX's lens block, so I had to zoom in more and lost the full SLR image. They may have a fix for this soon as well. you can sort of tilt the adapter with the allen screws on the mount but that didn't help my offset much. I don't know if this was a problem with my adapter or if its my HVX but I don't have the same problem with the others.
    Of course the Letus has a flip built in- wich I must admit I like a great deal even more than I thought I would.

    The SG Pro also feels well built and is a spinner which is an advantage in general but this makes makes it heavier than a Brevis. The advantage of a spinning adapter is that you can use it at much more varied f stops (5.6 and above) on the 35mm lens and at a wider range of shutter speeds and slow-mo. On the vibrators you have to watch out for static grain patterns, or dust and dirt appearing as out of focus shadows.
    On my camera I could not get full SLR frame size with the SGPro Rev2 but other people say they can. This might be a difference with the Rev3.

    The Brevis is by far the lightest and most well suited to handheld. You can put a wide angle lens on it and just go even without rails (as long as you have a flip monitor). Though it seems more lightly built I haven't seen any evidence that its flimsy or breaks down easily - Dennis brags about how he's dropped in many times with no damage.

    The Brevis has the advantage of replaceable diffusion screens to optimize vignetting for different lenses and to get different DOF and Bokeh. You can also change the speed of the vibrating motor. Letus claims they made it a closed unit to keep dust off the screen which is legitimate since opening the adapter to change screens does tend to pick up dust. However its not that hard to blow it off - if you catch it that is - and that's the dangerous part.

    With Brevis and SGPro ready to release flip units very soon, I'd say its an open horse race and they all make fine units. Dennis claims his flip only lose .2 stops of light, which will leave these tests aproximately unchanged if its true, but one or the other adding a flip will bring both adapters closer to the Letus' speed.

    All of these guys have done us a big favor. They work their butts off trying to improve these units and and we owe them all a big thank you. Maybe they'll have a relay system for 2/3" available soon as well.

    - Lenny Levy
    Last edited by Lenilenapi; 11-25-2007 at 10:21 PM.


     

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    #2
    Senior Member jenningsp's Avatar
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    the SGpro loses 0.6 of a stop with an F1.4 lens????? are you sure? i've heard it loses around 2 stops with a 1.4 lens.....
    Patrick Jennings
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    #3
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    What Letus?
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    Sorry - Letus Extreme. i'll change that in the post.

    jenningsp, I've heard all sorts of things about how much light these adapters lose. These tests were very careful and I saw similar results more than once - but they only happened on my camera.


     

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    #5
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    Thanks for posting your thoughts on the adapters.

    A few things I noticed that were off, but all in all it's just a different opinion really.

    I think the backfocus is already set for the Letus between mounts. I coudl be wrong, I thought Hien stated that you wouldn't have to adjust it.
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    On mine the mount came separately and I had to adjust it myself. There was no hard stop for it so it would have to be adjusted every time it went in. Perhaps others have had different experience, but unless there are different versions i don't know how you could just change mounts.

    Also they all send their units with the backfocus supposedly adjusted but my experience is that I've had to fix all of mine. My SGPro came second hand so that may have been someone else's fault, but I would check my back focus carefully on any of them - its a delicate adjustment. That's another issue I want to address later as I don't like many of the instrucrtions i've seen for how to set back focus.
    Hien didn't tell me about any other adjustments internally. As i mentioned I think shims would be a good and simple solution. Hien suggested that idea, but didn't provide any yet.
    He's been very responsive so my guess is they will develop a solution to both problems I had.
    He also asked me to send a photo of the offset in my viewfinder and i haven't gotten around to it yet.

    All these gusy are responsive. i had the same complaint about Backfocus on the Brevis last January and in a few months Dennis responded with his micro-collimation. We're very lucky.

    Lenny
    Last edited by Lenilenapi; 11-25-2007 at 05:40 PM.


     

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    DVXuser Sponsor Dennis Wood's Avatar
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    Thanks so much for the work you've put into this Lenny. It's nice to have someone else objectively (with numbers!) confirm our own findings.

    With regard to the the Brevis with the flip module, we've learned our lesson with camera's having off-center CCD or CMOS blocks. It can't be compensated completely at the achromat side, however our flip has been carefully designed to allow a 2nd point of adjustment. The bottom line is a usable, perfectly centered frame that is 10% larger then the SLR frame....so we can virtually guarantee that your fast lenses will have a greater field of view than if they were used with film :-) With the flip, the edge to edge sharpness has been tuned by an optical engineer, and inherent chromatic aberration with cameras like the XH-A1 is reduced a great deal. In fact, our testing is showing the Brevis/flip combination cleaner than the bare camera with Canon FD lenses.
    Last edited by Dennis Wood; 11-25-2007 at 06:30 PM.


     

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    The offset is on mines as well. I actually have to go to about Z82 to be able to use all of my lenses. It would be nice to have the offset corrected so that you could zoom through, but isn't the offset for every HVX a little different?
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    Yes it is different. we just rented our Letus to someone who had less offset and you've got more.

    Lenny


     

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    #10
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    It's good to know that at least all of the units are shipping out the same. Although my offset is probably similar to yours. With the 50mm on it's Z72, anything wider than that I have to go in more so I can't keep it at the same frame, unfortunately.

    It was the same with the M2, though, and in fact I had to be at about 82 the entire time so it's no big difference as far as my experience.

    I didn't know that the Brevis allowed you to back the zoom out that much more.
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