Loss of light when using a telephoto?

[leon_schnell]

The other day I was thinking about how light and lenses work, when I came up

with a question that nobody has been able to satisfactorily answer. Imagine

the following scenario: you take a photograph of a subject on a wide-angle

lens, and then take another photograph of the same subject at the same

settings, standing at the same distance, with exactly the same lighting, using

a zoom lens (ignoring lens defects and other external factors). Wouldn't the

subject in the photograph taken with the telephoto lens appear darker than the

subject (not the whole scene) taken with the wide-angle lens?

 

The reason I ask this is that if exactly the same amount of light is arriving

at the camera from the subject in both photographs, surely the telephoto lens -

which stretches the amount of light to cover a much larger area - will in

effect darken the overall image of the subject? I haven't gotten around to

testing this, but I figured that somebody here must know why this would or

would not happen.

 

Here's my rough attempt at sketching the situation in Paint:<div></div>

[garry edwards]

Leon,

 

I think that what you're really asking is "does the brightness of an image reduce with distance in the same way that light reduces with distance"

 

If so, then the answer is yes.

 

And no.

 

Yes it does, but the light reflected from the subject is concentrated into a smaller reflective subject with a wideangle lens and so the effect is identical.

[mike dixon]

<i>The reason I ask this is that if exactly the same amount of light is arriving at the camera from the subject in both photographs, surely the telephoto lens - which stretches the amount of light to cover a much larger area - will in effect darken the overall image of the subject?</i><P>

If the size of the aperture (not the f-stop, but the physical size of the opening) remained the same, then yes, the telephoto would underexpose the image relative to a wider lens. But if you are using the same f-stop, the physical size of the aperture increases with the longer focal length [f-stop = focal length / size of aperture]. In practice, if you are using the same f-stop with a wide angle lens and a telephoto, the exposure will be the same.

[Jochen_S]

No, that's why handheld 1ﾰ spotmeters can work with any lens which has f-stops engraved.

 

About the arriving amount of light: if x necessarry photons arrive during your shutterspeed the telephotolens with its bigger front element will catch more of them than a tiny 35mm in a P&S. - In fact the aperture counts, but today it's hard to take lenses apart to measure the sizes of the wholes.

[stephen hazelton]

Leon, you ask a very good question, but it has already been taken into account in the exposure system.

 

If you use the same lens, and back off from a subject, less light from the subject hits the lens. But the image from that lens is also proportionately smaller, so the intensity of the image remains the same. No problem there.

 

If you take two lenses of the same diameter but of different focal lengths, they will in fact give different exposure times at the same subject distance. However, use of the f/number accounts for this. So two lenses might have 25mm apertures, and one is an f/2 and one is an f/8- so one lens does in fact take a much longer exposure than the other. This is one of the reasons we deal with f/numbers rather than actual lens diameters or aperture diameters.

[pacorosso]

I do not understand completely thequestion �What do you mean when say "the same amount of light"? The exposure depends on the luminance of the scene. And the subject you are photographing does not change its luminance when you slect a narrow angle of vision.

On the other hand, there is differences in the way two diferents lenses capture the light. You can be confident in a number f lets pass through the same amount of light in a wide angle than in a telephoto. Of course there is differences, for example when shooting at very short or very long distances.

[nino]

Stephen H-

 

You've actually got it back to front with regard to your f/number comments. F numbers ARE actual physical measurements of the iris diameters. They are mathematical "F" computations taking into account only the diameter of the iris opening, relative to the front element.

The compensations for element light absorption is not taken into account (ie: a 50mm with only five or six elements will have less light absorption than a 300mm with twenty elements) that is why "T-stops" were invented for use in more high end lenses. A "T-stop" takes into account the light Transmission factors (ie physiaclly measures on the back element for each lens) as oposed to f-stop which is a mathematical.

Two different focal length lenses with T-stops of T8 for example will give exactly the same expoure on film, where as two lenses with F-stops set to F8 will probably give slightly different readings.

 

You will find that Cine lenses are ALL marked with T-stops. This allows for seamless intercutting of shots in a linear sequence with different lenses.

Such small exposure discrepancies are quite irrelavent when dealing with still-frame, as they can be easily tweaked (its only one frame after all), but when you have side to side comparison like inter-cut scenes in a movie, it does matter.

[helenbach]

Just a small point: f-stops are not calculated from the physical diameter of the iris, they are calculated from the diameter of the entrance pupil - which is effectively the image of the iris when seen through the front of the lens. You can make an approximate measurement of the diameter of the entrance pupil by measuring the diameter the iris appears to be.

 

Best,

Helen

[nino]

Yeah sorry, thats what i meant by "iris is measure relative to the front element". Thanks for clarification. Hope my post wasnt too convoluted. regards

Nino

[bill c.]

OK, I'll chime in here with my decades of photographic teaching experience and either clear it right up or so hopelessly confuse him that he'll never pick up a camera again. Either way, I figure problem solved.

 

First, you must understand what an F-number is. It is the ratio of the diameter of the opening in the lens (aperture) to the focal length of the lens. (NOTE: Lenses can have tricky optics-- the optical center of some telephotos is sometimes way out in front of the front element of the lens, and for very short wide-angles the optical center can be deep inside the camera, almost at the focal plane, so you can't measure the optical center of a lens by measuring the lens itself.)

 

So, suppose you have a common 50mm lens. If that lens is set to f2.0, then the diameter of the aperture is 25mm (this is with a simple lens, like I say, optics can be tricky, but this is for illustrative purposes).

 

Now suppose you have a 200mm lens. If you set that lens to f2.0, the diameter of the aperture would be 100mm. One way to think of this is that yes, the light coming from the subject is "stretched" over many times the area, but the aperture is the bigger by the exact same factor, and this compensates.

 

Another mind exercise is to realize this: a telephoto lens for one format can be a normal lens for another format.

 

A 200mm lens is just beyond normal for a 4x5 view camera. Now suppose you take a 200mm f 2.0 lens that will cover 4x5 (you can't, though, not really, because the front element for such a lens would be so darned massive that it would be impossible to mount, control, and transport, though there may be some industrial process lenses of this diameter). If such a lens did exist, and if you used it for a 35mm frame, it would theoretically give you the exact same exposure and image size on the film as the 200mm f2.0 telephoto lens made for the 35mm format. But if you stick it on a 4x5, it would give you an angle of view about that of a 50mm lens on a 35mm camera.

 

On the other hand, if you stuck the 200mm lens made for the 35mm format on the 4x5 view camera, you would get a small image circle with the subject the exact same size and exposure value as with the bigger lens, just less coverage.

 

In either case, on both lenses when used at f2.0 the aperture would (in theory) be the exact same size, the only thing that changes is the angle-of-view. The front element of the view camera lens would have to be much bigger because it has to take in a wider view. When it "necks down" to the optical center of the lens, that's where the aperture is, and that's where the f-stop is measured.

 

As was stated before, the exposure compensation is all taken into consideration by the use of f-numbers for the designation of aperture diameters-- you just have to understand what those numbers mean. In fact, the system was invented to prevent the type of problems your thought problem has enlightened you to.

 

Side note: the term "telephoto" refers not exactly to a lens of long focal length, but to a particular lens design, which "scrunches up" the light path at the front of the lens and "stretches it out" near the back. This enables a lens to physically be far shorter than it's optical length. This can be done if the focal length is long in relation to the area it needs to cover, which is why the front-element-to-film-plane distance on a 200mm lens for a 35mm camera can be made much shorter than on a 200mm lens for a 4x5 camera.

 

Hope that doesn't read like too much gobbeldygook.

 

Happy shooting. -BC-

[leon_schnell]

Thanks for the comprehensive answers (especially Bill Cornett). Surely there must be a point though where "bigger aperture" does not necessarily equal "more light". If there's only a single photon floating around, a small aperture and one the size of the Grand Canyon would be about as effective, I guess. But I guess this distinction is reached at light levels so low that our eyes and film/sensors wouldn't be able to register the difference anyway - just being tricky :P

[timsumma]

Instead of all the complex responses lets make this really simple. Set up a 4X5 view camera. Put a 90mm lens on the camera. Take two photographs. Cut a 35mm size image from the first negative, leave the second one uncut. Print both into 8X10 inch prints. Other than the view of the scene both will print correctly. There you have it, no change in light delivered; there is a different view presented.

A simpleton�s view of optics: You have two lenses for your camera one is the normal lens (35mm camera as example) of 50mm focal length. You have another lens of 100mm focal length (commonly referred to as a long lens). You take pictures with both all day long at f=4. At the end of the day you get that sinking photo feeling, something�s must be going wrong, we have all had it. These are different focal lengths but I have been shooting at the same f stop, one must be under exposed or the other overexposed, but in fact you find they are all fine. What�s the deal? For the exposure of f=4 on the 50mm lens the aperture is twice the size of the f=4 of the 100mm lens; or if you like, the f=4 of the 100mm lens is twice as big as the opening of f=4 on the 50mm lens. This is the Inver Square Law of Light in action. The light has to travel twice the distance in the 100mm lens as in the 50mm lens so the opening on the 100mm lens must be twice as large. Yes! This Inverse Square Law applies here on the planet surface also, it is just that the distance from the sun to what you are shooting at this place on the Earth is not great enough compared to that thing behind it that is 30 feet further away (but that is a difference we just can�t measure, unless of course your a physicist with lots of gizmos, but they never take pictures that excite other people except other physicist).