Lens optical piece diameter in regards to light

[sunny_nguyen]

<p>Hi all.<br /> I'm currently fishing around for a new lens (or if affordable, a couple) off www.keh.com for my DSLR, and here are the lens I've temporarily rounded it down to, before choosing one of these or other lens, based on suggestions, after getting some answers solved.<br /> - Canon 50mm f/1.8<br /> - <a href="http://www.keh.com/camera/Canon-EOS-Non-Mfg-Zoom-Lenses/1/sku-CE09999054843J?r=FE">Tamron 19-35mm f/3.5-4.5</a><br /> - <a href="http://www.keh.com/camera/Canon-EOS-Zoom-Lenses/1/sku-CE07009040811J?r=FE">Canon 20-35mm f/3.5-4.5 USM</a><br /> - <a href="http://www.keh.com/camera/Canon-EOS-Zoom-Lenses/1/sku-CE079990339230?r=FE">Canon 28-135mm f/3.5-5.6 USM/IS</a><br /> Anyway, my problem here is this - I do both daylight people photography of friends, sunset/sunrise photography and night photography in the city. With a tight budget of $300 and the affordability of used lens off KEH, I really don't know which to choose from, and I'm really hoping to get a suitable lens soon because Sydney's currently hosting to a spectacular light show which I've waited all year for.<br>

<br /> What I want to know is, if the lens optic piece diameter (the glass at the front where filters are attached) affects shutter speed at a given aperture size. For example, if I had a 50mm lens set to f/3.5 with its 52mm diameter, and a Tamron 19-35mm set to f/3.5 with its 77mm diameter, if the 19-35 will ultimately get a faster shutter speed than the 50mm given it logically should allow for more light due to the larger optic piece.<br>

<br /> I had my friend do a quick test with his lenses (a 50mm with 52mm diameter and a 18-105mm with 67mm diameter) and he reported that the 50mm at f/3.5 actually produced a brighter image with the 18-105 set to 50mm at aperture 3.5, and that the camera in aperture priority set both to identical shutter speeds. But with a small sample, we're not concrete on the outcome.<br>

<br /> My point in asking is that I'm a bit reluctant on placing down $274 for the 28-135 with its IS because of its smaller aperture size of 3.5 compared to the 50mm's 1.8, because as I said, I do a fair bit of low-light photography. I don't know how much faster a shutter speed would the IS help on that lens, because if the 28-135 with its 72mm optic piece at f/3.5 can be as fast as the 50mm at f/1.8, or f/3.5, with IS enabled, then I'll happily settle for that lens. But I don't really want to get just the 50mm for the larger aperture because as useful as it is in low-light, I'm not sure if it'll fit my purposes if I ever wanted to zoom in without manually cropping later.<br>

<br /> I'm sorry if this is a tad bit confusing, but I hope you understand what I'm trying to ask. But thanks in advance for any tips from anyone :)</p>

[kelly_flanigan1]

<p>(1)For an object that can be magnified; like a person; house the *Fstop* determines the speed<br /> <br />(2) For a point light source like a star; the *Front Diameter/Aperture* determines the speed</p>

<p>Both a 100mm F4 and a 50mm F2 have a 25mm aperture, The F2 is faster for case (1); Both are the same speed for recording a star. For a Nebula the F2 is faster.</p>

<p>A 250mm F10 lens has a 25mm Aperture; so does a 500mm F20 lens and 50mm F2 lens; all are the same speed for a point light source. All if on a telescope will have the same faintest star to be seen.</p>

[oscar_van_der_velde]

<p>Light falloff (vignetting) may explain differences in brightness you get from different lenses or zoom settings for a given f-stop. Also, there may be some error margin in the aperture.<br>

I don't know what the nature of this light show is, but if it's fireworks, you don't need a fast lens at all since wide apertures will overexpose it (wash out) even at ISO 100. I've had great results at f/8-f11.</p>

[bonyari]

<p>Hi Sunny,</p>

<p>I don't think there is a direct relationship between the diameter of the front element of the lens to the light intensity. To explain the phenomenon that you & your friend had found, it's probably due to light loss in the lens. Each time the light goes through a glass element in the lens, some of the intensity is lost (that's why we pay a lot of money for lenses that have incorporated fluoride glass - which minimize light loss). So as you can see, the more elements a lens has, the more light loss it has - or the darker the image you'll get from it.<br>

I didn't check for the specific lens designs, but I suspect that the 50mm has fewer elements in the lens than the zoom lens does (which is reasonable). </p>

<p>I observe the same phenomenon with my 2 lenses: 100 f/2 and 100 f/2.8. The image I get from the 100 f/2 is always brighter (same aperture, same shutter speed). And it's b/c what I said above, I think.</p>

[joe_fikes]

<p>To first order, the only thing that counts is f-number. That is, any lens set to f/3.5 gives the same "brightness" (it's actually photons per area) on the sensor when viewing the same scene as any other lens set to f/3.5 would. And that doesn't depend on focal length, front element diameter or anything else. As has been noted above, there are small, second-order effects such as vignetting and the overall lens transmission that also comes into play. There is also a tolerance to which the lens aperture actually is at f/3.5 when the camera says it is. But those things are small relative to the overall f-number. </p>

<p>Joe</p>

[philip_wilson]

<p>Your test was probably not at F3.5 for the 18-105. I am not sure what lens this is but in general most zooms that go to a ratio over 3x are not constant aperture. Indeed F3.5 to F5.6 is a common range. What this means is that an 18 - 105 F3.5 to F5.6 lens at 50mm is probably somewhere between F4.5 and F5.6. You will find that the EXIF data will actually show you the aperture used - not the one marked on the lens.<br>

As others have said the lens has to be in proportion - but a crude rule of thumb is that the size of the aperture is related to the F stop and the lens FL as follows (for 35mm)<br>

An Aperture in mm is approx Focal length in mm / fastest F stop.<br>

Thus a 200mm F2 lens needs an aperture 100mm across, while a 50mm F2 lens needs one 25mm across.<br>

If we take two zoom lenses - say the 24-70 f2.8 and the 16-35 f2.8 we would find that the 24-70 neds an aperture of 70/2.8 = 25mm while the 16-35 needs an aperture of 35/2.8 = 12.5mm. This is why the barel of the 24-70 is fatter than that of the 16-35. however, when you look at the front element the 16-35 has an 82mm diameter while the 24-70 is only 77mm. this is because the horizontal angle of view of the 16-35 at 16mm is 97 degrees while the 24-70 at 24mm has an angle of view of 74 degrees. thus the wide angle lens needs to focus light from a wider range of angles and the design Canon has chosen is to use a wider front element. There is no hard and fast rule for this but in general the wider the lens or the faster the lens the bigger the front element. In Canon's case the old 16-35 had a 77mm front element while the new one has an 82mm front element. In addition AF lenses tend to have a larger filter diameter as the lens barrel is wider due to the AF motor so camera manufacturers often use a wider filter diameter. For example the old Canon FD 50 F1.4 was a 52mm filter diameter while the EF50 F1.4 is a 58mm filter. Similarly the old FD 80-200 F4L was a 58mm filter while the EF 70-200 F4L IS is 67mm but both lenses let in the same light.</p>

[mark_poseley]

<p>Until you press the shutter both lenses will be open to the largest aperture possible; they will stop down to F3.5 to take the shot. You don't say what the maximum aperture of your friend's lenses are, but I suspect the 50 mm is larger than 3.5.</p>

<p>If the 50 mm is F1.8 and the zoom is F3.5 max then the 50 will be much brighter in the view finder until you take the shot. But F3.5 is F3.5 irregardless of the lens used. If this was not the case it would be very difficult to take consistent exposures...</p>

<p>Does that make sense?</p>

[richard_oleson]

<p>The diameter of the front element is a function of the design of the lens; it is typical, for instance, for a wide angle lens to require a very large, negative element at the front to receive oblique light rays and bend them toward the rest of the lens that sits behind. This does not increase the light gathering power of the lens, it is there for optical reasons.</p>

<p>If the design of the 2 lenses were identical, then the physical diameter would be significant; but of course the optical design of a 50/1.8 is not going to have much in common with that of a wide-to-tele zoom.</p>

<p>Actually, the diamter of the REAR element of a lens is a much better indicator of its relative speed than that of the front. It's still not perfect, but in lenses that have the rear element in roughly the same location relative to the film (in 35mm SLRs, this might be in the 20-100mm range or so), there is some correlation between rear element diameter and speed. But it's easier just to read the number off of the aperture ring.</p>

[sunny_nguyen]

<p>Hi everyone, thanks for your explanations. I haven't got too much time to reply to this individually, I'm sorry, but from what I get here, the general consensus seems to be that I should get the 50mm lens if I'm working with low-light photography, right?<br>

I'm just worried about the limitations I'll face with the 50mm compared to the 28-135mm with IS. From an uninformed-until-recently mind of myself, I thought that with IS compensating by 2-3 shutter stops, I'd get a shutter speed fast enough that blur isn't so much of an issue as it is without IS, especially when taking photos in the evening with low light. The 50mm at f/1.8 will obviously allow a lot more light in and hence make taking photos faster and the blur problem eliminating itself to an extent (not sure how much, I've only ever done night photography at f/3.5 on my stock 18-55 lens), but I'm still wondering then, if f/3.5 with the IS should help enough for me to consider getting that lens instead.<br>

Once again, thank you all for your detailed responses and explanations on the matter, I've learned a lot so far - now for the five-day wait until my pay is deposited, until then any suggestions and such would be greatly thankful for :)</p>

[sarah_fox]

<p>Sunny, the IS won't allow you to shoot at a higher shutter speed. What it will do is to stabilize the image, so that you can shoot at a slower shutter speed without seeing as much blur from the motion of the camera. (You use IS for hand-holding, not for tripod use.)</p>

<p>If you shoot moving objects (e.g. people who are moving around), then the IS won't help you. If you shoot stationary objects (e.g. trees that are not blowing in the wind), then IS will help you quite a lot. If you develop really good technique, the older IS versions, such as on the 28-135, can give you a 4 stop shooting advantage. The later IS versions, such as on the 18-55, can give you around a 6 stop shooting advantage.</p>

<p>Here's Yorktown's Coleman Bridge shot at a 1 sec shutter speed with an 18-55 IS:</p>

<p><img src="http://www.graphic-fusion.com/phcolemanbridge01.jpg" alt="" /></p>

<p> </p>

[sarah_fox]

<p>Oops, again...</p>

<p>With lots of breath control and meditative relaxation (being one with the camera, and such)...</p>

<p><img src="http://www.graphic-fusion.com/phcolemanbridge01sm.jpg" alt="" width="700" height="467" /></p>