Fstop to Tstop

P

Park Edwards

Veteran
How do I match all my lens to the same stop? For instance I have 3 lens I use.
50mm F1.4
85mm F1.8
20mm F2.0

Obviously, the F2 on my 50mm wont match my 85mm or the 20mm, so what do I need to do to have them all match up? Or, what is the formula to doing so?
 
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The T-stop is in effect a corrected F-stop--corrected for light loss from absorption by lens coatings, etc. For modern prime lenses the difference between F-stop and T-stop is negligible. Large zooms and older lenses may show a greater difference between the theoretical F-stop and the actual T-stop due to light loss.

Is this your question??
 
My understanding of the matter is the same as Roberts.

With the technology of lenses, the difference is so minimal that it does not matter that much; especially with the power of being able to color and exposure correct.

So use the same F-stop for all of the lenses. And if there is the most minute difference, do a quick fix in post. Go ahead and just test this theory with RAW still images.

--

If I'm wrong, I'd like to know.
 
I'm not talking about stills. I'm talking about the D90 movie mode. The difference is very significant. I'll just have to take out each lens and measure it that way. Also, when trying to get the same ISO/Shutter with each lens will be impossible unless you match the same amount of light that enters with each lens.
 
The only way to do it is to use your eye on the LCD and try to match the exposures as closely as possible. Having a laptop nearby and checking each shot side by side would help. The limitation being if you wanna shoot in the dark at 1.4 you'd have to stick to the 50mm lens for the entire scene or brighten the set to shoot with the slower lenses. I'm more concerned about the different coatings on each lens affecting the chroma.
 
I guess I'm not making myself clear. In order to match shutter speed with different lens, you have to match amount of light that enters the lens. So if my establishing shot was with the 50mm f/1.4, stopping down to F4, using the light trick to adjust shutter/iso and then went into a wide shot, I could not match it unless I knew exactly what stop on the 50mm matched what stop on 20mm/85mm.
 
I get what you're saying, but as far as I know there isn't any accurate/repeatable way to do it. If we had shutter and ISO control then I suppose we could measure the amount of light entering each lens at different f-stops with the help of a light meter.

Our young zacuto apprentice mentioned that the zeiss lenses are all calibrated at the same f-stop. But who has the money to fork out on a set of zeiss primes?
 
How do you know the t-stops are different? The difference should be tiny, and either way how are we supposed to know? We don't even know what glass it is you have? Are you sure it's not simply a field of view problem with your light source? You can always test t-stops by shooting a few bracketed stills, as suggested.
 
By tiny i mean much less than a stop, probably less than a third, so even if you found out it would be very hard to use that info in practice. It's not like f/4 is t/5.6 on one and t/8 on another but probably t/4.5 or so on both.
 
@ O.P

Sorry mate but your understanding of f/stops and t/stops and exposure is confused.

Go back to basics - an exposure is an exposure.

Dont over complicate this.

It has nothing to do with the "stop" on the lenses apart from getting it correct ( per meter or waveform or eye).

EXPOSURE has everything to do with the amount of light, the ISO/ASA and the shutter speed. Then one considers the desired effect/look /DOF from the exposure (ie over/under and deep/shallow DOF).

One does not have to worry about f or t stops on lenses matching. They just do in the practical world. e.g If the meter says f5.6...use f5.6 on the 25mm or the 200mm lens.
 
Matt you shot your feature with multiple nikon primes right? How did you go matching exposure by eye? And did you find you had to do any color correction to match the shots from lenses that have different coatings?
 
you're making yourself perfectly clear, but it doesn't necessarily make sense. :-) i think i know what you're trying to do but i think you're asking the wrong questions. read my two post above and i'm sure i can help you out. the difference in t-stops between lenses is so small that if you see a significant difference it must be due to something else.

/matt
 
squig said:
Matt you shot your feature with multiple nikon primes right? How did you go matching exposure by eye?
Click to expand...

Since we shot under 50 hz lights we got 1/50 consistently by the waving around method and then it was just a matter of adjusting the f-stop slightly until it looked the same. I had a very experienced dp so it went well. For outdoor we simply exposed for the sky, just barely blowing it out, which meant consistent exposure as long as the clouds didn't change too much. I'll start color correcting in july, will let you know more then. :-)
 
I just shot some stuff under a dim 50hz fluoro using the "waving" method. One shot was quite good, grainy but that's what I wanted. The next 2 shots looked the same on the lcd but I didn't notice the D90 had pushed up the ISO to an unusable level until I looked at the shots on my mac.

I'm hoping either there's a firmware update or a new model nikon by the time I start shooting the feature, otherwise I'll have to lug around a monitor to be sure the ISO isn't too high everytime I lock exposure.

Did you experience this or were you using a monitor on set while locking exposure? I have to stress that it was a very dimly lit scene shot at f/1.4 so I was really pushing the D90 but a lot of my feature will be shot in dark environments which the D90 is more than capable of with the right shutter and ISO settings.
 
I dont even own a D90 but I think he is trying to maintain the same shutter speed across all lenses that he is using. Sounds like he is trying to lock exposure before he does a shot or the auto exposure control will change his shutter speed. I remember reading the D90 adjust ASA and then shutter speed to control exposure. Or am I wrong too?

Like everyone is saying the diffence between the F stop and T stop is pretty small BUT I think you could do what your trying to do by setting up a test card and light it at f4. Then see what shutter speed you camera sets itself at. Then put the new lens on (with same f stop set) and see what shutter speed the camera changes too with the same amount of light. The difference in shutter speed will tell you how many 10th of a stop they are different. Look up the difference in the american cinematographers manual. Hope that helps.

On a camera that has all manual controls like the HVX200 you would tweak the shutter speed to compensate for the difference in light loss between lenses. So if you 50mm 1.4 at f2 was 3/10ths a stop different than your 28mm at f2 then you would tweak the shutter accordingly. (I use a 35mm adapter by the way). You'll have to look up a chart for the details.
 
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