Why is the front element of a telephoto lens larger than a wide angle lens?

A wide angle lens has a wide angle of view, therefore it would make sense that the front of the lens would also be wide. A telephoto lens has a narrow angle of view, so it would make sense that the angle would be smaller. However, this is the opposite of what one can see on real lenses.

Why is this the case?

Answer

That is a generalization that is not always true.

The front element is not only sized to accommodate the field-of-view but also for the maximum aperture. Since aperture is measured as a fraction of focal-length, a longer lens requires a physically larger aperture to reach the same F-stop.

For example:

• The Nikkor 200mm F/2 lens requires an aperture 100mm across and you can see from the spec that the lens diameter is 124mm.


• The Nikkor 35mm F/2 lens requires an aperture 17.5mm across and you see that the diameter of lens is much smaller.



• The Nikkor 300mm F/4 lens requires an aperture 75mm across, so although it is longer than the 200mm F/2, does not need to have such a large front element and its diameter is 90mm.


• The Nikkor 14-24mm F/2.8 is an ultra-wide lens with a diameter of 98mm although its aperture only needs to be 8.6mm across (24/2.8) but it ends up much wider to accommodate the field-of-view.

Contradiction in information resources on capturing UV light with a digital sensor

I encounter a contradiction.

Many resources state that a digital sensor are unsensitive to UV light. Even wikipedia, "However, newer photographic film and digital cameras are highly insensitive to UV wavelengths." http://en.wikipedia.org/wiki/UV_filter. Digital Photography Review goes even to the point that a digital photographer won't even need it on a beach (an UV-light rich area). "digital sensors are not and hence do not need UV filters in even bright sunlight". http://www.dpreview.com/articles/8049091537/the-uv-filter

While other resources state that digital sensor are sensitive to UV light. Are digital sensors sensitive to UV?

Can someone clarify this contradiction?

My prognosis is that a digital sensor is sensitive to UV light, but sensor manufactures include an UV blocking system on the sensor. But I don't know if that's true. I do know that IR light is blocked through a IR-absorption glass on the sensor itself.

Is my prognosis correct?

Answer

Most digital sensors are sensitive to UV to some degree, and most have UV/IR cut filters installed over them to keep that sensitivity from throwing off visible light colors. However, removing the cut filter to let in the full spectrum, or using a pass filter and long exposures, doesn't really solve the issue of allowing the camera to shoot in UV, because the majority of lenses also have UV cut coatings on them. Few lenses are suitable for UV shooting, and the ones that are tend to be expensive and rare (e.g., the UV-Nikkor 105 f/4.5), which is why this is a much rarer and more esoteric form of photography than IR shooting. It's most commonly done by forensic photographers.

See: All You Ever Wanted to Know About Digital UV and IR Photography, But Could Not Afford to Ask

equipment damage - Is it ok to breathe on the front glass element of a lens to fog it and clean it?

Nikon specifically recommends not breathing on the glass elements to clean them as harmful acids in the breath can damage the coatings on the glass.

However, I've read on many other websites that it is OK to clean the lenses in this manner.

What are the potential pros and cons of doing it?

NOTE: Shortly after this question was posed Nikon removed all references to the issue of breathing on Nikon lenses from the page at the above link.

Answer

Ever since I posted this question here, this has been featured on 2-3 websites:

Although I'm not sure if this was a direct result of posting this question here.

Nikon has now updated the support page and removed the statement where it said that breathing on the glass could damage the lens coatings due to presence of harmful acids in the breath.

Canon 700d - lens does not move when focusing?

I have a canon 700d. I have seen other 700d's, and when they focus the lens automatically moves in and out. My one doesn't. Why would that be?

troubleshooting - Why is the sky in photos always too white?

Why do I never seem to get a clear picture of the sky during the day, even with white puffy clouds in the sky? It always looks over exposed and whitened out? I am using a Nikon D3100, no lens hood, manual setting.

exposure - What's a good ad-hoc replacement for a gray card?

I know that a real gray card is the best thing to use to determine exposure. But I'd like to know if there are other naturally available features that can be used in a quick and dirty way. Are fields of grass or road tarmac appropriate?

The reason for asking is that I suspect that my new used (manual focus) lens is exposing incorrectly using my camera body. I don't have access to a gray card, but I do have access to lawns and pavements, nicely and evenly lit by a cloudy sky.

Answer

Grass is generally considered to be 18% grey. I would guess if you shoot in grey scale mode and your exposure is said to be correct, it should come up 18% grey.

There's also the sunny 16 rule, which is on a sunny day, shoot at 1/ISO shutter speed, f16. This should produce a correctly exposed picture. You can check to see if this results in a correct picture.

Both of these were tricks I was taught back in film days.

aperture - What's the base f-stop when counting an ideal f-stop for lens?

I've read a lot around this site that an ideal quality of the lens is achieved when using f-stop roughly 2 stops slower than the fastest value of the lens.

Firstly, I'm not exactly sure what "2 stops" mean when it comes to f-stop, but this isn't really important in what I'm about to ask. What I'd like to know is what is the base f-stop when counting this value.

Let's look at it this way: If I have a prime lens with f/1.8, then obviously I'll be getting the ideal value from f/1.8, let's say after 2 stops I get f/2.8?

But! What in case of zoom lenses, for example 18-55, which have f-stop f/3.5 - f/5.6. Should I take f/5.6 as my base value, or does the base value depends on the real fastest value at the exact focal length?

Therefore, if I set the lens to 18 mm, their fastest aperture is f/3.5 and thus the ideal quality is at f/5.6 and when I set it to 55 mm, their fastest aperture is f/5.6 and thus the ideal quality is at f/8 ? Or is it f/8 for the whole focal range ?

Answer

Real fastest value at the exact focal length.

But, this is just a rule of thumb — it's not necessarily exactly two stops in every case.

For the question of what two stops mean, see What does f-stop mean?. In short, each stop is approximately the-square-root-of-two times the previous one. That means half the light is allowed in (which is why the seemingly-weird series of numbers was chosen). Two stops is doing that twice, which conveniently works out to simply being doubling. Two stops from f/2 is f/4, and and two stops from that is f/8. Or, starting at f/1.8, two stops is f/3.6.

The test results comparison tool used by Digital Photography Review is interesting for looking at this. Try this test of Pentax's 18-55mm zoom, which like most kit zooms goes from f/3.5 at the short end to f/5.6 zoomed out.

You can see that at 18mm, the sharpness improves as you go from f/3.5 to f/4 to f/5.6. At f/8, it's more even from center to corners, but doesn't get any sharper. And beyond that, it drops off.

Then, change the focal length to 55mm. You'll notice that with this lens, overall sharpness goes down from 18mm at f/5.6. At f/8, it picks up a little bit, and a little more at f/11, and then back down again at f/16.

So, that fits with the two-stops guideline pretty well. Same for the Canon kit lens, and Nikon too.

With the Pentax DA 15mm f/4 Limited test, they've got test results in third-stops, and there you can see that f/7.1 is no better than f/6.3, and f/8 is worse — so that's only 1⅓ stops. On the other hand the DA★ 55mm f/1.4 shows peak sharpness at f/4.5 — closed down by 3⅓ stops. This doesn't reflect a problem with the lens, just different priorities in design. The Nikon 50mm f/1.4G is similar, with peak sharpness at 3⅔ or 4 stops down. However, the entry level Nikkor f/1.8G fits the "two stops" guide quite well, peaking at around f/3.5.

It's also worth noting that this test is primarily concerned with sharpness, because that's easy to measure. Other image quality and rendering characteristics are also affected. Vignetting (light falloff in the corners) gets better the more you stop down, and it's usually gone after two stops. And bokeh quality is usually improved by stopping down, too — generally, lens bokeh is nicer/smoother stopped down a bit, but of course it's less visible since you have greater depth of field. Plus, shape of the aperture blades will become visible in specular highlights, which is a side-effect unrelated to stopping down per se — some modern lenses have rounded aperture blades to make that not be a concern.