Dumb question of the day

<p>When you have a zoom lens that is, let's say for example....2.8 - 4.0 over 24mm-85mm.... How is the aperture segregated within that range?<br />I know I should know that, but I really never paid much attention to how small an aperture change you can make or the camera makes itself....but I am wondering if that is a set up in the camera when in aperture priority or it is just in half stops, etc.<br />2.8 > 3.1? > 3.4?> 3.8? > 4.0<br />or are there other graudations in there if you are shooting in aperture priority. For example, when you take the lens off and have it in your hands, the aperture level allows INFINITE RANGE of aperture..</p>

<p>Like I said, I know I am going to feel dumb when I get your responses.</p>

<p>For information, I have a Nikon D300.</p>

<p>Possibly dumb answer of the day: I don't think (mind you, it's been years since I used a variable aperture lens) there is a fixed/ specific ratio of how your aperture changes as your focal length changes. I'm sure it has to do with the movement of the crystals internally, but I can never remember it being a precise ratio. After all, say you have a 18-200 f/3.5-5.6 lens - chances are at 18 it will be 3.5, but how would you attempt to express (mathematically) the relationship when at 23 it goes to f/4 but then at 25 it goes to f/4.5 which it retains for the next 60mm or so?</p>

<p>On your D300, you can go into your menu system and dictate whether you want your AP mode control to dictate aperture in full stops, half-stops, or third-stops. I typically use the one-third setting, since I'm picky. So, that means the changes go from 2.8 -> 3.2 -> 3.5 -> 4, the way I'm set up.<br /><br />I'd have to do some testing to see how the presence of a variable-aperture zoom impacts that.</p>

<p>Follow up: I just did a test. Left a D300 in 1/3-step AP mode. Mounted a variable aperture DX zoom (the 18-70, if anyone cares), and set the aperture to f/3.5 (that lens's widest, at 18mm). As I altered the focal length, the camera displayed f/3.5 -> f/3.8 -> f/4 -> f/4.2 ... so those are smaller changes than the 1/3-stop parameter would call for on the command-wheel data entry level. I believe you'll also find those smaller-than-1/3-stop changes kicking in if you're in SP, or full-on P modes, where the camera happily splits aperture hairs to get the exposure just right.<br /><br />So, the onboard software thinks in finer aperture setting resolution than you can directly tell it to use in AP. This doesn't surprise me, particularly. I don't need to dial in a deliberate AP change that's smaller than a third of a stop, that's for sure.</p>

<p>@Matt Lauer -</p>

<p>That's interesting, but did you happen to figure out at which focal lengths it changes?</p>

<p>@Wade Thompson -</p>

<p>I don't think there's any way to know how the aperture is segregated for any individual lens without testing it. In other words, I don't think there's a linear scale that you can apply to every lens since there are several moving elements in zoom lenses.</p>

<p>RS</p>

<p>On a variable aperture zoom, there are no discrete aperture steps as you zoom. In other words, it's continuous rather than in steps. As you zoom, the aperture is not actually changing at all (the blades, that is). The only thing that changes is how much light gets through the lens as the focal length changes. The camera only shows a different aperture number that corresponds to this.</p>

<p>It's different if you actually change the aperture setting on the camera. Then of course, the steps are specific, and unless the camera allows different settings (as yours does), it's usually in 1/3 stops.</p>

<p>Richard:<br /><br />I've observed this behavior on a few different variable-aperture zooms, and they all seem to have a different curve, even when they overlap on focal lengths.<br /><br />For example, I know that the 18-200 and the 18-70 each behave differently from 18 to 70mm, in terms of how quickly they stop down. It's going to depend on the optical recipe, and what sort of other compromises they had to make in the design to get the size, weight, etc. Your best bet, for a given lens, is just to test it empiracally, and get to know the lens.</p>

<p>The maximum aperture at different focal lengths from Thom Hogan's 18-200 lens review. Some lenses are at their "fast" aperture for much of their focal length range while others quickly get to the "slow" aperture. It depends on the lens design.<br /><br>

18mm f/3.5<br /> 24mm f/3.8<br /> 35mm f/4.2<br /> 50mm f/4.8<br /> 70mm f/5<br /> 135mm and higher f/5.6</p>

<p>So, does that also mean logically that at 18mm, for example, the lens/camera combination will expose the photo at only 3.5, 3.8, 4.2, 4.8, 5.0, 5.6 and nothing in between?</p>

<p>It will, Wade, if you're in AP mode and <em>tell </em>it to do so, because the data input device (the command wheel) and the software quantize to those points. But, as I mentioned above, you'll find your actual as-used aperture somewhere between those 1/3-stop values if you're shooting in shutter priority mode (or certain program auto modes) and the camera is deciding on an aperture to use after it has taken shutter speed and ISO into account. It will then land you an aperture value wherever it likes, even if it's between the AP-based natural 1/3-stop setting values.<br /><br />But if you're going to tell it what to use, it will only do what you (can) tell it.</p>

<p>The changing aperture of a variable aperture zoom is a function of the continuous change in optics as the lens is adjusted, as Pierre says - in the same way that the effective aperture on a macro lens changes continuously as you focus closer. I suspect the reported apertures are merely the closest maximum apertures that the lens is telling the camera about - the camera needs to know about the maximum aperture so that it knows how much to stop down by moving the aperture lever to get the aperture that the user actually requested. On a related note, most cameras can't move the aperture lever continuously - if I select f/8 on my D700 and hold down the DoF preview button, the brightness of the viewfinder varies significantly as I zoom in and out with my 28-200, because it can't compensate for the changing maximum aperture. I believe the D3 series <i>can</i> control the aperture lever dynamically.<br />

<br />

After sounding authoritative, a stupid follow-up question: if you have a variable-aperture zoom that's not "G" (and all the variable aperture zooms I own <i>are</i> G, so I don't know), how does the aperture ring interact with the variable aperture? Is it just that the faster apertures are clipped at the long end of a zoom, are they physically locked out, or do you have to apply some kind of zoom factor, as you do with macro distances?</p>

<p>@ Matt Laur -<br>

<br />Thank you...you confirmed my suspicions.<br>

<br />RS</p>

<p>@ Andrew: the maximum aperture on my 28-105 D ise not physically locked out when zoomed out. You can still set the max at 3.5, even though that's not what you will get. There are two dots to indicate which aperture you've selected and what it's equivalent is zoomed out to 105. At least that's what I believe the second dot is for. Of course, in aperture priority, the camera knows what the max aperture is for a given focal length is and won't let you select wider than what is possible. Something I find interesting though, is that it also knows what the minimum aperture is for each focal length and will allow you to use apertures that cannot be selected physically. The minimum aperture on the ring of the 28-105 is f22, but if you use the command dial, you can select up to f29 when zoomed to 105mm. Similarly, the minimum aperture of my 28-80G is f22 @ 28mm, but you can go up to f40 @ 80mm.</p>

<p>I for got to add a pic of the aperture ring. Also, the 28-105 has a macro mode, so the second dot may be the effective aperture in macro, rather than @ 105mm. </p><div></div>

<p>Ahah! Thanks, Cory - that was going to have bugged me for ages. Now I check the other variable-aperture lenses, I see they all have two marks, and some have them colour coded with the zoom length marked on the lens. That's even a vaguely logical system! Cheers - that's two bits of historical photographic trivia I've learned today (the other is that the LCD illuminator button is an alias for the M button in manual mode on the Eos 630; yesterday I finally worked out how you put film in a 5x4 camera - ain't education wonderful?)</p>

<p>ain't education wonderful?<br>

Yes and that is exactly why I come here and become such a sponge. GREAT WEALTH of experience among many of you.... PLUS.... the excitement of new people coming in and learning.</p>

<p> </p>

<p>aperture is a ratio of of the amount of light a lens can allow in to the max possible any lens can. ie a lens marked 1:2 is a f2 lens which allows 1/2 the light that a f1.0 allows. f stop is calculated by focal lenght/iris opening, both usually in mm. ie my canon 600mm f4 lens is just over 6 inches/150mm in diameter in the front, which divides 600/150= f4 or 600/f4=150mm opening. with a zoom with a fstop/certain size aperture opening, as you zoom it effectively changes. a 2mm opening at 24mm= f12, while at 50mm its f24.</p>

<p>Er, Michael? The statement about f stop calculation is correct, noting a) that it doesn't matter what units you're working in so long as the entrance aperture size and focal length are both defined in the <i>same</i> units, and b) that the entrance aperture might be virtual; for a lens that can focus to infinity, the front element will always be at least the size of the entrance aperture - for wide angle lenses it's typically much larger. It's also true that an f/2 lens admits half the light of an f/1 lens (ignoring the actual transmission of the lens and the - likely significant - fall-off at f/1). But the ratio compared with "the max possible any lens can"? I'm afraid that's fiction - I refer you to the range of f/0.95 lenses available and the f/0.7 and faster exotica. Just heading off any confusion.<br />

<br />

Not that this is what Wade was asking about - he was talking about variable-aperture zoom lenses. As an aside, it's usually (always?) the case that the lens's effective entrance aperture is largest when a zoom lens is at its longest setting. Since stars, being (effectively) point light sources, have a brightness that depends on the absolute size of the entrance aperture rather than the relative f-stop, it's a curio that stars will show up better with - say - a 28-300mm f/3.5-6.3 lens zoomed to 300mm and f/6.3 (for a 48mm aperture) than with the same lens at 28mm and f/3.5 (an 8mm aperture); this is why focal length matters much less to a telescope than its aperture.</p>