F stops or T stops?

[Helen B]

***

The f-stop shown on the lens should be used when calculating depth of field: an f1.4 lens may in reality be an f1.8 but it will have an f1.4's depth of field - or should have.

Hi Nicholas,

That's got me puzzled. I guess that it isn't a typo for T1.8 because we've already mentioned the use of f-stops instead of T-stops for focus calculations. Why would a lens that is marked as f/1.4 but is actually f/1.8 have the depth of field of an f/1.4 lens?

Thanks,
Helen

 

[Dan Fromm]

Macro lens such as Luminar and other technical lens also use T stops .

My Luminars don't have t/stops, their diaphragms are scaled in exposure time compensation factors. 1, 2, 4, 8, ... My Neupolars' diaphragms are scaled in mm, i.e., the scale read's the stop's diameter.

 

[Nicholas Lindan]

Hi Nicholas,
That's got me puzzled ... Why would a lens that is marked as f/1.4 but is actually f/1.8 have the depth of field of an f/1.4 lens? Thanks, Helen

You know, now that I think about it, that's a good question. Don't really know, TTH, but that doesn't effect my propensity to potificate on matters about which I know not (it's a Guy thing, like never asking for directions):

Some of the additonal attenuation seen in a camera's meter reading is due to greater vignetting of the corners throwing the whole meter reading off, assuming a semi-averaging or matrix meter. But there is additional attenuation in the center of the field when wide open.

So, the depth of field should also be attenuated some. Wide open there are two parts to the attenuation: the attenuation seen at any stop, and the additional attenuation of the peripheral rays for whetever reason. It is the additional peripheral rays that are responsible for the larger CoC and if half of them (WAG) aren't arriving then the CoC will have fall-off at the edges and act as a smaller CoC.

If the lens is marked 1.4 and:

f/stop t/stop

f1.4 1.7 - 1/2 stop attenuation
f2.0 2.2 - 1/4 stop
.... .... - 1/4 stop

Not knowing any better, I would split the difference of the additional 1/4 stop of attenuation and figure that at wide open the lens is behaving like a
f1.4 + 1/8 stop = f1.45 lens for depth of field purposes.

This is only for the center of the image. The corners have additional attenuation because the exit pupil is cut-off by vignetting. Whether this reduction in effective aperture causes an additional decrease in dof at the corners I really don't know.

 

[Joe VanCleave]

Angle of View vs. Metering

Solarize, I would think that, in practical terms, the biggest difference in actual use between an f2.8 wide angle lens and an f2.8 telephoto is the angle of view of the two respective lenses as compared to the angle of view of the light meter in question. If TTL metering is being used, I would expect only a negligible difference, if any.

Obviously, external metering will present the greatest difference in results, as the wide angle lens matches the angle of view of most handheld meters better than it does a telephoto lens.

And spot metering externally would present a different issue; potentially there would be very little difference between both lenses, if the same scene area were being spot metered, and that scene area was predominant in both respective fields of view.

Another big difference between an f2.8 wide and an f2.8 telephoto is the cost!