different lenses and different f -stop value with the same camera

[h_s1]

<p>I was wondering if somebody can help me understand this discrepancy I noticed in two lenses regarding the aperture needed in each case for a scene.</p>

<p>On a Canon T-90, if I use Canon fd 50mm f/1.4 lens and set Tv=1/125s, the meter wants Av=f/9.5 or f/11 for a particular scene (pointed to the northern blue sky for this test). But if I use a Tokina ATX fd 28mm-85mm f/3.5-4.5 lens, the meter wants Av=f/6.7 or f/8 for the same settings and scene. How can I explain this? Is there something the problem with the Tokina lens? Or is the transmission of light different in both lenses (if yes, is the common and as pronounced)?</p>

<p>Thanks.</p>

 

[jimmckinnon]

<p>Are you setting the zoom at 50mm, and framing to take in precisely the same elements? Or, is it possible the zoom is including more dark areas or fewer light areas in the scene? f/x.x should equal f/x.x for exactly the same lighting falling on exactly the same subjects, <em>if </em>there are no additions or subtractions to the scene.</p>

[h_s1]

<p>Well, I was pointing the camera toward the sky and I was using partial metering at that time. I wasn't sure I was doing this right (since the f/stop is supposed to be the same for the same scene or illumination), so I tried this a few times and got the same difference.</p>

<p>I will check again tomorrow though and report back.</p>

<p> </p>

[h_s1]

<p>Well, just did a small test with my monitor screen. At Tv=1/20s, here is what I get:<br>

50mm f/1.4 lens: f/5.6<br>

50mm f/1.8 lens: f/5.6<br>

Tokina 28-85 zoom: f/4.5 (within some zoom range around 50mm)<br>

35-105mm fd f/3.5 zoom: f/4.5 (within 35~70mm at least)</p>

<p>What gives?</p>

<p> </p>

[steven_endo]

<p>I suspect it is because zoom lenses have more glass so less light transmision requiring a larger f-stop.</p>

[peter_e]

<p>Is the picture taken with the Tokina at f/5.6 darker than the same image taken with the Canon lens at f/5.6?<br>

If yes, then Steven may be correct. If no, your TTL meter does not meter correctly with one of the lenses, presumably the Tokina. I posted a few days ago on TTL metering with non-brand lenses.</p>

[h_s1]

<p>I shot a slide film with the two lenses a few days ago and am yet to scan it. But from the slides, I don't see any perceptible difference. I have shot a negative roll this weekend and will find out in a few days time how it went.</p>

<p>I checked the meter again at Tv=1/500s and get f/11 for both lenses when pointed to the overcast sky. But if I point lenses to a wall in the room, for Tv=1/30s the camera wants f/4.0 for the 50 mm lens and f/3.5 for the tokina lens set at approx. 50 mm focal length. Looks like the meter shows same readings at smaller apertures but wants half or 1/3 stop more open for the Tokina at wider apertures.</p>

<p> </p>

[rick_janes]

<p>I have a hunch that the marked aperture values on lenses are calculated values more than anything, thus the small difference your meter is seeing in actual use (see also t/ stop).</p>

[h_s1]

<p>Rick, I am not clear what you mean by the markings on the lens. The camera is working in Tv mode. I set the shutter time and it tells me what aperture to use. The lens marking do not come into play at all!</p>

<p>I agree with the T-stops though, as I mentioned in my original post.</p>

<p> </p>

[ashbrown]

<p>I'm no expert on this topic, but have noticed the same thing. My f1.4/50mm meter reading is about half a stop faster than most my other lenses (allowing for the different lens speeds of course), while the meter reading for my f2.8/20mm is about half a stop slower than my other lenses. Maybe this is due to a combination of manufacturing variations, plus the manufacturer's rated lens speed being rounded to the nearest traditional f-stop rather than being an actual measured speed for each individually made lens. Perhaps zoom lens speeds are rated a little optimistically by the manufacturers too.<br>

Therefore, my 50mm should probably be rated up half a stop to f1.2 instead of f1.4 (good news!), but my 20mm should probably actually be rated down half a stop to f3.5 (bad news, since it was far more expensive than the 50mm!).<br>

It would be interesting to know if other FD owners who have multiple copies of the same lens (seem to be a few on this forum) can report variations within their own collection of the same lens.</p>

[barryreid]

<p>Just a thought but...<br>

The Tokina has a variable aperture. If the lens mechanism only translates the f-stop number for it's maximum value to the camera then at 85mm the light reaching the meter is less therefore for any given shutter speed you will get a larger f-stop at 85mm than at 28mm.</p>

<p>The same may apply to the 35-105 if it's not an f/3.5 constant?</p>

[feanolas1]

<p>The camera gets the max aperture through a small protuberance on the mount; the longer it is, the bigger the aperture is. At f/8, it is flush with the mount (case for my mirror 8/500R). Let say there is a 1/10th of error on this value, it will not register on the full aperture value as it is rounded to the nearest half aperture. However, it may be just enough to make the difference when measuring an intermediate value: say you measure 5.1, this is rounded to 5.6 by the T90; however if you have an error on the full aperture of 0.1, it would evaluate 5.0, which would be rounded to 4.5.<br>

I think Barry is right in saying that a variable aperture should make this especially noticeable. And since there is no adverse effect on the pictures, it means it is just a problem of display.</p>