A Few Basic Questions

New here; hope it's okay to ask a few questions that I couldn't find answers to.

1) Given two lenses of similar quality, both stopped down to the same aperture (e.g. f/5.6) for a particular shot, is there any advantage to having the "faster" lens (larger maximum aperture)? Or, is it just because faster lenses are usually of higher quality?

2) Is there any inherent advantage to a lens that uses 72mm (or 77mm) filters over a lens that uses 52mm filters? (Assuming the same sensor size, of course.) For example, on my T2i, if I had two 100mm f/5.6 lenses, would the one with the larger filter somehow gather more light through the f5.6 aperture? Or, are there other advantages to the larger filter size?

3) Possibly related to question #2, (again, with the same sensor size) do all lenses of similar focal length have similar field of view? Would every type of 50mm lens (on the same camera) produce an image with exactly the same boundaries? Would a 50mm prime lens have the same field of view as my 18-55mm zoomed to 50mm? Or, would an 18-270 also have that same field of view when zoomed to 50mm?

Thanks - I really appreciate any help with these questions.

cacawcacaw wrote in post #10222742
New here; hope it's okay to ask a few questions that I couldn't find answers to.

1) Given two lenses of similar quality, both stopped down to the same aperture (e.g. f/5.6) for a particular shot, is there any advantage to having the "faster" lens (larger maximum aperture)? Or, is it just because faster lenses are usually of higher quality?

It's because the faster lenses are made to work with much larger apertures and subsequently must produce an acceptable image quality at a larger aperture. So when you stop them down they increase in quality with a head start so to say. Not all lenses though - it's not an absolute rule.

The filter size is a consequence of the front element size. Larger front element = more light entering the lens. If both lenses have the same maximum aperture then the amount of light getting to the sensor should be the same..

Yes, in theory. In practice field of view may vary with your focus point. Cheaper zooms can have huge variations between minimum focus distance (MFD) and infinity.

cacawcacaw wrote in post #10222742
1) Given two lenses of similar quality, both stopped down to the same aperture (e.g. f/5.6) for a particular shot, is there any advantage to having the "faster" lens (larger maximum aperture)? Or, is it just because faster lenses are usually of higher quality?

The aperture stays all teh way open until you actually take the pic. The wider aperture will give a brighter image in the viewfinder and provide more light for the AF system to work with.

The light coming through should be the same, but the larger filter size should be much less likely to have vignetting in the corners.

50mm is always 50mm. Within manufacturing tolerances, adjusted for marketing copy and measured at maximum focus distance.

So your 18-55 might actually be a 17.5-57 or an 18.8-52 or some such. The marking on the zoom ring might not be 100% accurate. And the field of view will change as you focus closer.

Hey, thanks for the quick and accurate answers. I can't help but laugh about getting two great answers within 10 minutes, and from North Virginia and Sweden. Couldn't have done that forty years ago!

To take my question another step further, I'm assuming that the label given to lenses with mirrors or doublers (or even fish-eyes?) is based on the focal length of a conventional lens with a comparable field of view.

Thanks again for the answers - clears up much of the confusion I had while talking to friends about their new lenses.

In modern lenses the focal length is the "effective" focal length and is not tied directly to the actual physical size of the lens. There are some technologies like mirrors that can be used to make the lens much more compact while maintaining the same focal length, but there are trade-offs in image quality.

Fish eye lenses are a different thing. The Canon 15mm fisheye provides a much wider angle of view than the Canon 14mm non-fisheye. http://the-digital-picture.com …-Fisheye-Lens-Review.aspx has some good samples.

An obsevation regarding FAST lenses- very non sientific but it's what happened. Late last year a friend and myself were shooting rutting Deer (with cameras!). He was using a 200-400 f5.6 at f6.3 to 7.1 and 800 to 1600 iso as the light reduced. He was struggling to get "safe" shutter speeds (640th or better). I was taking pictures of the same animal with a 400 f2.8 (non is) at mainly f7.1 to f9. I was using iso 200 to 400 any had VERY few shots BELOW 1000th sec. Same apeture same amount of light? I don't think so.
Must admit I don't know what the sience says but the exif data confirms. I knew there was a reason I was carring around an extra 4.8 kilos of lens.

Lenses stopped down do perform better than wide open. But, it is not correct to assume that a f/4 lens at f/5.6 performs inferior to an f/2.8 lens stopped down to f/5.6 !!! Some lenses are at their best 2-3 f/stop from max, but others perform at their best only 1 f/stop from max. The Canon 70-200 f/4L IS has better MTF scores than the Canon 70-200 f/2.8 L IS at f/5.6

Same FL on the same body provides same AOV. However, lens are marketed with a nominal FL, but lab measurements have shown lenses to be shorter or longer than rating! A so-called 50mm lens might truly be 51.7mm, for example, while another model of 50mm (different max aperture) might be 49.5mm, so the AOV would be different for comparison of these two lenses.

John, that's the type of situation that had me questioning the difference between lenses. But in your case, I can think of another variable - the amount of glass that the light has to travel through. If I recall correctly, a lens multiplier will cost at least one f stop, presumably because of the extra layer of glass. It seems that your 400mm prime lens might have fewer layers of glass than your friend's 200-400mm zoom. And, I'd also assume that the glass in your prime lens is probably higher quality - would it also be "clearer"?

Interesting example. I wonder what other variables were involved. Were you both using full-frame cameras?

edit: I just did a quick search and found that I should have referred to the "layers of glass" as lens elements, which are assembled into distinct groups. The prime lenses are far more complex than I thought, but I never came close to realizing how many elements it takes to build a zoom lens.

cacawcacaw wrote in post #10223437
If I recall correctly, a lens multiplier will cost at least one f stop, presumably because of the extra layer of glass.

A telextender (or 'multiplier') change the FL of the lens, without a change in the aperture size of the lens. Since f/stop is the ratio of aperture size to FL, the effective f/stop of the combination (lens+telextender) becomes smaller. Slower f/stop is not due to 'extra layer of glass'

Thanks for explaining that Wilt. I mistakenly thought it was more like the situation with Image Stabilization, where you virtually "gain an f/stop" by being able to use a slower shutter speed. So a telextender actually changes the physical dimensions used to calculate the f/stop.

Am I way off base with my line of thinking? Are some lenses "less efficient" because their numerous elements filter out some of the light? Are other lenses "more efficient" because of their optical quality?

With the quality of glass used in modern lenses there is almost no light loss in each element and any light loss is compensated for in teh design.

On the other side of it, lens companies do not add additional glass elements just for the fun of it. More lens elements are generally an indication of a higher quality lens because each element is being added for a reason.

There does appear to be some minor differences in transmission...I set a camera on a tripod aimed at a uniform wall, and found in one test that a Canon zoom and a Tamron zoom would both result in a 1/3EV difference in the meter reading. That is not to say that they were actually 1/3EV different in transmission, but the values might have been 'on the fence' and cause a higher reading from one lens and a lower reading from the other (where there might have been 1/6EV real difference)

Old lens tests from Popular Photography would list a T value (transmission). I have a copy of results of Olympus OM lenses of different FL, and the T value was between 93% and 95%, too small of a difference to really factor in for exposures!

Good answers here; one other clarification.

The main advantage of a "faster" lens is mostly that it CAN open to a wider aperture, thus making it "faster". An f/2.8 lens @ 5.6 and an f/5.6 lens @ 5.6 would give you the same performance, assuming all other qualities are equal.

Hi, it has been a rule that a lens performs best and is sharpest at 2 stops below its maximum aperture,
the filter size doesn't come into it. Beeclose.

Beeclose wrote in post #10223712
Hi, it has been a rule that a lens performs best and is sharpest at 2 stops below its maximum aperture,
the filter size doesn't come into it. Beeclose.

Hi, you obviously have not read the entire thread, especially psot #7.