Canon 7D minimum aperture

<p>I just got a Canon 7D, it's great and everything but I can't figure out why sometimes I'm unable to make the aperture setting go as low as it normally does. In other words, most of the time it goes down to f3.5, but sometimes it only goes down to f4.5 or f5. I'm using Canon's 18-135mm f3.5-5.6 lens. <br>

I'm guessing this has something to do with its designation as "3.5-5.6," meaning the lowest aperture is always somewhere between those values, but I haven't figured out the conditions that change the minimum aperture setting. Anyone got a good explanation? Thanks.</p>

<p>Hello - The f3.5-5.6 refers to the maximum or widest aperture (smaller numbers are larger apertures, larger numbers are smaller) through the range of the lens. This means the maximum aperture of f3.5 is the case at the 18mm focal length. The maximum aperture at the 135mm focal length is f5.6. In between the 18mm and 135mm focal length the maximum aperture will be between f3.5 and f5.6 and moves toward the f5.6 as you get closer to 135mm. Maximum fixed aperture lenses are typically much more expensive and much larger. For example a 70-200 f2.8 lens retains it's maximum aperture throughout the range of the lens. There is nothing wrong with your lens.<br>

I hope this helps!<br>

Alex</p>

<p>Seriously, people who don't understand this stuff before buying a dSLR really...don't need that dSLR...aren't ready for that dSLR...</p>

<p>Alex already answered your question but I want to say something.<br>

Not everybody knows everything and you have to learn things by asking questions.<br>

A teacher of mine once told the class; there are no stupid questions only stupid answers.</p>

<p>... and stupid people too.</p>

<p>All of us were beginners once. If you can't respond civilly to the question of a current beginner, perhaps it would be better to simply not respond at all. Just sayin'</p>

<p>Dan</p>

<blockquote>

<p>All of us were beginners once.</p>

</blockquote>

<p>Are you sure?</p>

<p>Some, like Mozart, seem to have started as experts, while others never made it as far as "beginner". ;)</p>

<p>An old story about a famous archaeologist professor went as follows:</p>

<p>Student- Why did you give me an F?<br /> Professor- Because there isn't any lower grade.</p>

The aperture is a circular opening in your lens that lets light through.

 

The f number describes the DIAMETER of that circular opening. f is the focal length of your lens in its current zoom

position.

 

When f is 18 mm, f/3.5 is about 5 mm, approximately the diameter if a pencil. Imagine a round hole of that size in the

middle of your lens. It's not very big, and it fits easily inside your lens' casing.

 

When f is 135 mm, f/3.5 is about 38 mm, or about an inch and a half. This is approximately the diameter of the handle of

a shovel. A hole of that size is too big to fit inside your lens. So for this lens, f/3.5 is impossible when f = 135. f/5.6 is a

smaller hole that will fit inside your lens. The engineers restricted your maximum aperture at the telephoto end of your

lens in order to keep the overall size and price of this model down.

 

This principle explains why some lenses are physically large. The 70-200 mm f/2.8 needs to be able to support an

aperture of 200 mm / 2.8, or about 70 mm. That's a hole of almost three inches in diameter, which is why this lens is so

large.

<p>[[seriously, people who don't understand this stuff before buying a dSLR really...don't need that dSLR...aren't ready for that dSLR...]]</p>

<p>oh the irony. and by irony i mean: what a joke. </p>

<p><a href="../wedding-photography-forum/00Zefd">http://www.photo.net/wedding-photography-forum/00Zefd</a></p>

 

<p>Great find there Rob. Hopefully some people will learn to think a bit before posting rude comments</p>

<p>This has to come up at some point when you get a DSLR. I don't think anyone ever really gets educated about variable apertures while using a P&S. Is everyone supposed to do hours and hours of tedious reading and research on the internet and in the library before their allowed to buy a DSLR? That sounds like fun; people buy things before they <em>really</em> need it all the time. Do you think that guy in the Ferrari really needs 600 hp or understands how his paddle shifters work? No, but now and then its nice to get something before you're an expert on the subject.</p>

<p>Ross: I and many others have used Busch's Canon guides to learn camera basics; <a href="http://www.amazon.com/David-Buschs-Canon-Digital-Photography/dp/1435456912?ie=UTF8&tag=thstsst-20&linkCode=as2&camp=1789&creative=390957">he has one for the 7D</a> that's probably worth your time and money.</p>

<p>The ironic thing is that many zoom lenses that have 'fixed' max f stops (such as my 70-200/2.8 whose aperture can open to 71.4mm) are mechanically able to achieve ~f1 at their widest focal length. The microchip in the lens closes the aperture down when at wider focal lengths to preserve the IQ (which would get pretty bad pretty fast!).</p>

<p>So the 24-70/2.8 is actually, physically, capable of being a 24-70/f1-2.8 (same w/ the 70-200/2.8) ... mind blowing, huh!</p>

<p>Wow, just checked back in for the replies, thanks for the helpful responses, didn't realize this would cause such a controversy. For the record, I understand what aperture is, how the number scale works, what is going on inside the lens physically, how it relates to exposure/depth of field etc., I've just never encountered lenses that changed aperture with focal length (learned photography on an old film SLR, just getting back into photography now). Anyway, thanks again for explaining that to me, and as for the people who were born knowing everything, I think they need to find something slightly more interesting/important to get worked up about. Cheers.</p>

<blockquote>

<p>The ironic thing is that many zoom lenses that have 'fixed' max f stops (such as my 70-200/2.8 whose aperture can open to 71.4mm) are mechanically able to achieve ~f1 at their widest focal length. The microchip in the lens closes the aperture down when at wider focal lengths to preserve the IQ (which would get pretty bad pretty fast!).<br />So the 24-70/2.8 is actually, physically, capable of being a 24-70/f1-2.8 (same w/ the 70-200/2.8) ... mind blowing, huh!</p>

</blockquote>

<p>This isn't how it works. Think of a virtual aperture size and not physical. As you zoom in, the virtual size of the aperture increases when you look through the lens. There is no microchip that closes down the aperture.</p>

<p> </p>