Fstop to Tstop

[mattsand]

the concept of f/4 = f/4 and f/5.6 = f/5.6 is so incredibly basic and ingrained, I can't see how anyone would disagree with it?

If you wanna calculate dof or discuss the size of the aperture in relation to the focal length sure, but I thought we were talking about exposure?!? Exposure is determined by the t-stop, which is usually close to the f-stop but never quite, and varies between lenses.

 

[Shooter]

Notwithstanding the limitations of still cameras trying to be proper movie cameras where the user has full control over ISO /exposure , shutter speed etc...

In real world photography ... the difference between a t stop and an f stop is not worth chasing or losing sleep over.

So many other variables like the way a man holds his meter to the light and the accuracy of the device (ever seen 3 meters on a set all match to point 1 of a stop).

I have a box full of Digital and analogue Spectras and Sekonics plus spotmeters to back this up but I have never lost a shot because of a bad exposure. Half the time when doing aerial shoots I guess the stop from a baseline reading ("eye-ometer").

Fix it in post?

Anyway ...what is the formula (you mentioned) to determine a t-stop from an f-stop.

 

[Barry_Green]

In real world photography ... the difference between a t stop and an f stop is not worth chasing or losing sleep over.

Again, agreed with Shooter. The difference between T-stop and F-stop is usually very minimal. Almost pedantic to even argue over. Have you seen any prime lenses that are marked in both f-stop and t-stop? I can't think of any. I had an old Cooke 20-100 that was f/2.8 and t/3.1, which means the t-stop was about 1/4 stop different. That's a lot. Yet it's a tiny amount. And that's on a zoom; again I can't think of any prime lenses that bother to mark both. I looked up the Ultra Primes, most of them list an f-stop of 1.7 and t-stop of 1.9, but that's not how the lenses are marked, they don't even bother to mark them that way. That's just in the technical details page.

So yes - if you want to argue the finest of points, it's possible that a t-stop will be ever so slightly slower than the corresponding marked f-stop, even on a prime. But does anyone worry about it? Have you calibrated your light meter differently based on every specific lens you've got? If that's what the discussion is, then sure, have at it, but in the real world I've never seen anyone care. If there's enough of a difference between the t-stop and the f-stop that it might actually make a difference, the manufacturer marks the lens accordingly.

 

[evsdan]

I'm with Bary and Shooter here. I think the main issue is NOT is the same amount of light reaching the sensor with a 50mm at an f4 as an 85mm on an f4 (which has already been sort of answered; yes within a small amount of variation not worth discussing). Yes 2 T stops would be better, but if you only have F stop markings you should be OK. Would this be ok in the world of photography? I don't know I would think so. I know for a fact that 1/4 or even 1/3 exposure errors in the film world are pretty much not worth noting (most of the time...) Hell half the DPs I know over expose by as much as 1 1/2 stops as a matter of course. Of course almost every cinema lens I have seen has T stops instead of f stops.

The issue, I believe, is with the D90 in particular and its constant changing of shutter speed even when the lenses are set to the same stop. Basically once you lock down the ISO and the f-stop on the lens, the camera is changing the shutter speed to compensate. Is this the issue? Can you carry around a grey card and a dimmable light to fill the lens FOV up to get the exposure to where you want it to be?
Does this have to do with the camera's metering mode? The matrix metering on the FOV of an 85mm is going to be different than on a 50mm since there are more shadow and highlight areas (theoretically) on the 50mm.

 

[Shooter]

I am now really curious to hear from the OP Car3o.

What lenses do you have? Are they stock Nikkors?

I have a friend (top fashion photographer and film director) who bought a Canon D5xx. His disillusionment with his purchase is palpable due to the same issue I feel is running behind this thread. His workaround is to use a Nikkor to Canon adapter with Manual Nikkor lenses (his and mine). What a shame considering what fantastic (read expensive as well as quality)Canon lenses he has in his arsenal.

He is not concerned at all wth the t/f stop debate raging here.

I am stunned that these cameras despite the amazing images they are capable of...have a market with anyone.

 

[Park Edwards]

I'm not sure if this D90's video mode is different from still or what, but I'll do my best with some video later demonstrating what I'm talking about. First i'll try to write out a scenario. In one of my threads I went about finding out shutter speeds and the trick to do that is by stopping down the lens to a certain stop with a closet light in front, locking the AE, then opening the stop up. So, with my 50mm f1.4, I stop the lens down to f/4 lock the AE and open it up to f/2.8. Now onto my 28mm f/2.8, when I do the same EXACT thing, I open it to it's widest, it's not even close to my 50mm at 2.8. It's darker and the ISO is higher. The 50mm is much brighter. Nikon AI lens. That's all I know when it comes to brand.

 

[Shooter]

Thanx Car3o

You have confirmed the issue is really within the camera "operational area" and not the lenses. We do understand the issue and I sympathise with you (and my mate). It is seriously compromising.