Is F4 on the 50mm F1.4 the same as on the 70-200mm F4?

[homalgo1]

I'm looking at a second hand 70-200mm f4. I'll be mostly shooting indoors and I think I'll have enough light but im not sure. I have a few prime lenses already, if I set my aperture to f4 on my 50mm 1.4 to test the quality for example, will that be the same as the f4 on the 70-200mm? I'm new to photography so I'm not sure if the settings between lenses are universal. Thanks

[wq9nsc]

The level of illumination through the lens will be the same with both set at f4 (within insignificant rounding error since no two varieties will be exactly the same).  Of course the field of view will be different from the same location with the two different focal lengths (especially at the 200 end of the f4) so when making your tests keep in mind what will be in frame with each lens.  Depending upon what you are doing, the 70-200 f4 could give you much better results if you are having to crop significantly with the 50mm while the longer zoom would allow you to more completely fill the sensor area with the desired final image.  This is important if you are now having to capture images from further away than optimal for 50mm.

However, I have NEVER regretted buying "fast" glass and a good lens is one that you will use for a very long time and even with modern sensors a full f stop (from 2.8 to 4 is a full stop) makes a significant difference in noise.  So make sure that you aren't spending "almost enough" to get the lens that you want.  As more people abandon Canon EF mount bodies for R series bodies, more excellent glass will become available at a good price.

I still have the non-IS 70-200 f2.8 I bought back in 2005 and it gets used from time to time on one of my camera bodies even though I primarily use the current version I bought a couple of years ago.

Rodger

[kvbarkley]

The short answer is that the settings are the same, and interchangeable. Setting both lenses at f/4 should yield the same image with the zoom set at 50mm. Go ahead and try it.

As usual, there are caveats, so if you want the *real* answer keep reading.

To a first approximation yes. As far as all the optics equations and depth of field calculations go.

But because of all the extra glass and reflecting surfaces, the zoom will transmit less light than the prime. That is why cinema cameras are rated in "T" stops to give the actual amount of light that is transmitted.

However, as a practical matter, don't worry about it, it does not affect photography at our level.

[ebiggs1]

Perhaps and easier way to demonstrate this is to lay a 8 1/2 x 11 sheet of white paper on the table and draw two circles. One 50mm and one 200mm. Is the light different on either? There will be more total light on the 200mm circle but the exposure will be the same.