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Gary Kinghorn

Gary Kinghorn

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If I had two lenses. One is F2.8 across the range, one is F4. The filter size is the same on both.

If I shot at F4 with both lenses using the same ISO on the same camera, in identical conditions would the shutter speed be the same in AV mode for both?

Oh and the final distance is the same on each.
 
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Gary Kinghorn said:
If I had two lenses. One is F2.8 across the range, one is F4. The filter size is the same on both.

If I shot at F4 with both lenses using the same ISO on the same camera, in identical conditions would the shutter speed be the same in AV mode for both?
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Yes it should be
 
If everything is equal (and that includes EXACT framing of the scene), the meter should give the same exposure, so with the ISO and aperture pre set, the shutter speed should be the same.
 
Brilliant thank you.

To move on a little if everything else was as before but one had 77mm filter size and the other 58mm would the results still be the same?
 
Yes. If the user set aperture is the same, the results should be the same. The filter size is usually related to the aperture in that a faster lens will usually have a bigger filter thread than a slower one but once the fast lens is stopped down to what the slower lens can achieve, they'll behave the same (in theory, the faster lens should give better IQ at that point though but that's a different subject to what the meter says!)
 
1st. going back to your first post it is Impossible to have the same size filter.

2nd. f4 is F4 - thats not lens specific...
 
To clarify..
F4 is the amount of light that reaches the sensor. It is the same amount of light no matter the lens or iso.
 
Thanks people. You know when something goes round your head for a fair period and eventually you aren't sure who you are or where you live. This was one of those moments.
 
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Can I ask where filter size came in to this at all? Just intrigued what your thinking was :-)
 
Mr Will said:
Can I ask where filter size came in to this at all? Just intrigued what your thinking was :)
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Presumably the larger the required entrance pupil diameter, the larger the physical size, the larger the filter?
 
Diving Pete said:
1st. going back to your first post it is Impossible to have the same size filter.
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You had better have a word with both Canon and Nikon then...

Canon 24-70mm f2.8 L MkI = 77mm, Canon 24-105mm f4 L = 77mm
Nikon 24-70mm f2.8 G = 77mm, Nikon 24-120mm f/4 G VR = 77mm

o_O

FWIW though filter size has no standalone impact on the settings. If the lens is an f/4 then it will have a diameter necessary to let in the amount of light to get it to that aperture. Generally that will increase with the focal length of the lens - although the MKII version of the Canon 24-70 f.28 L is an 82mm (+5mm from the MKI)- probably as a result of a redesign to make it sharper edge to edge, and to improve quality for higher MP bodies which challenge the resolving power of older lenses.
 
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Diving Pete said:
1st. going back to your first post it is Impossible to have the same size filter.
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Hi, You are mistaken there, I have in front of me two Nikon lenses both of which are the same focal length but the maximum aperture is different by one full F-stop. Both take 52mm filters.
 
Gary Kinghorn said:
If I shot at F4 with both lenses using the same ISO on the same camera, in identical conditions would the shutter speed be the same in AV mode for both?
Click to expand...
Yes.

But.

In the movie industry it's very important to know that they can get *exactly* the same exposure with different lenses, and it turns out that f-stops aren't quite good enough for this. The aperture value - e.g. f/2.8 - is a physical measurement of the aperture, so it measures how much light comes in, but it doesn't measure how much light actually reaches the sensor. That can vary from one type of lens to another, depending on the number of elements, nature of coatings, etc. So high-end cine lenses often quote something called T-stops, which are essentially f-stops corrected for transmission loss. Thus one f/2.8 lens might be T/2.9, whereas another might be T/3.1. (But if they aren't cine lenses you probably won't be able to find that out.) Set both those lenses to f/2.8 and you won't get the same exposure. A very similar exposure, sure, but not exactly the same.

This might all seem a bit abstruse, but of course it can affect anyone shooting video with a DSLR.
 
The filter sizes can differ depending on the manufacturer spec and the curved glass used. It the minimum size for a f4 lens is say 58mm there is no reason this cant be bigger the aparture hole will just remain constant
 
Filter size relates only to the physical dimension of the front of the lens structure.
Most manufactures these days try to keep this the same throughout a range of lenses, or at least to a minimum number of sizes.
manufacturers also take into account any possible vignetting effect when using a filter and lenshood.
 
I'm a bit late on this, but anyway...

If the target is an evenly lit grey card, then with fixed ISO, I think f/4 should result in the same shutter speed irrespective of the focal length. That said, the point made above on T-stops is a good one. I've no idea how much difference in light transmission is likely between different camera lenses. I do know it's quite different for binoculars that have same basic specs, but have different glass..

Filter size is a bit of an irrelevance, for example Sigma 50mm f/1.4 has a 77mm filter, Canon 50mm f/1.4 has a 58mm filter thread.. It just comes down to how the manufacturer chooses to design the lens, including the point already made about vignetting etc..
 
StewartR said:
Yes.

But.

In the movie industry it's very important to know that they can get *exactly* the same exposure with different lenses, and it turns out that f-stops aren't quite good enough for this. The aperture value - e.g. f/2.8 - is a physical measurement of the aperture, so it measures how much light comes in, but it doesn't measure how much light actually reaches the sensor. That can vary from one type of lens to another, depending on the number of elements, nature of coatings, etc. So high-end cine lenses often quote something called T-stops, which are essentially f-stops corrected for transmission loss. Thus one f/2.8 lens might be T/2.9, whereas another might be T/3.1. (But if they aren't cine lenses you probably won't be able to find that out.) Set both those lenses to f/2.8 and you won't get the same exposure. A very similar exposure, sure, but not exactly the same.

This might all seem a bit abstruse, but of course it can affect anyone shooting video with a DSLR.
Click to expand...

I wasn't aware of the T-stop system - really interesting. I suppose there could also be subtle colour changes too depending on the optical characteristics of the various bits of glass that differ in each of the lenses too?
 
Furtim said:
I wasn't aware of the T-stop system - really interesting. I suppose there could also be subtle colour changes too depending on the optical characteristics of the various bits of glass that differ in each of the lenses too?
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Absolutely.

A rather extreme example perhaps, but one of my mid-1970s Canon FD lenses (a 35mm f/2 SSC) uses radioactive Thorium in the glass.

Over the years the thoriated glass has gone visibly yellow, which needs correction for digital colour use. The yellowing is reversible if you leave the lens exposed to direct sunlight for a week or two.

However, when I'm shooting with black and white in mind (which is quite often on either film or digital) it means the lens has a built-in yellow filter. Perfect! :)
 
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Anyhow, different manufacturers will 'tune' their lenses colour so that they're matched across the range should you switch between two different lenses by the same maker. This is/was rather more important when using film, especially slide film where you have no opportunity to tweak the colour during printing (or digital PP).

Yashica C/Y mount lenses are generally thought to be cooler than Contax-Zeisss lenses for the same mount, for example.
 
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