Another Lens question

[travis_mccormack]

<p>Sorry for the stupid beginner questions ,but I am just trying to learn.<br>

I have a Minolta XD-11 and I have a 50mm MD Rokkor-X 1:1.4 lens and I<br>

just got a wide angle Minolta MD 28mm 1:2.8 lens . <br>

My question is when I compare the two lens I realized that the back of the 28mm<br>

lens opening is alot smaller than the 50mm lens? Is this going to expose the same amount of the negative or <br>

is the 28mm lens not going to expose the whole negative? I have used the 50mm before and <br>

it works great. I just want to make sure that my new 28mm lens is going to work with my camera <br>

correctly?<br>

I have attached a couple pictures of the two lens side by side. <br>

Any help would be great.<br>

Thanks<br>

<img src="http://i172.photobucket.com/albums/w19/Travisvt82/GetAttachment-3aspx-2.jpg" alt="" /><br>

<img src="http://i172.photobucket.com/albums/w19/Travisvt82/GetAttachment-2aspx-2.jpg" alt="" /></p>

[ken_kuzenski1]

<p>Travis, that's perfectly normal. The f-stop depends on the diameter of the opening AND the focal length, so a shorter focal-length lens will have a smaller opening at the same f-stop as a longer lens. Give it a try and you'll almost certainly find it is fine. Those old Minoltas were great cameras in their day, and they can still make beautiful images--hope you have fun with yours!</p>

[accystan]

<p>I have those lenses, got them in the 70's and used them on my Minolta SRT-101. They are both excellent lenses. I think that the smaller diameter of the 'film end' of the 28mm lens has to do with design of the lens and the internal optics. The whole film or sensor area is still covered. I believe the angle of view of the 50mm is in the order of 45 deg. whereas that angle on the 28mm is 65 deg. As a disqualifier - I'm not sure if 'angle of view' is the correct term and it's different if it's measured diagonally or across the frame.<br>

Cheers,<br>

Dave D</p>

[travis_mccormack]

<p>I might have not been very clear, sorry!<br>

I think you are talking about when you change the aperatue how the blades inside create a smaller or <br>

larger opening....<br>

What I am talking about is if you look in the first picture the diameter of the actual back lens is about half the size.<br>

Maybe you are talking about that and I am just misunderstanding you,but either way let me know.<br>

Thanks so much</p>

[andylynn]

<p>That's just how the lenses bend the light. Don't worry about it - both lenses project a large enough image circle to cover a 35mm film frame.</p>

[travis_mccormack]

<p>Great I really appreciate the help</p>

[mark_drutz]

<p>F/2.8 is the same exposure on any lens. The f-stop is determined by the diameter of the lens opening and the focal length, so the longer the focal length the wider the lens opening has to be to give the same f-stop. That's why the opening is wider on your 50mm than on your 28mm when both are set to f/2.8.</p>

[travis_mccormack]

Ok thanks, I still am not sure if I'm being clear , because I'm

not talking about the opening changing when I change the F-stop

I am talking about the actual lens diameter on the back of the lens being

different sizes , no matter what f-stop I'm at(because the lens diameter never changes)

sorry for being complicated I guess I can't think of a good way to explain.

[accystan]

<p>Just as an aside.... the f-stop is not just an arbitrary number. If you have a 50mm f/1.0 lens (and such lenses exist - Canon used to market a f/0.95 lens) the diameter of the aperture is 50mm. That's what f/1.0 means. When it is stopped down to f/1.4 the diameter of the aperture reduces such that the area (and hence light gathering ability) is one half of that lens at f/1.0<br>

This is why, when you get to longer focal lengths, (200mm, 300mm), f/2.8 is often the best (and most expensive) you can get - because the aperture and hence the front element gets massive.<br>

Cheers,<br>

Dave D</p>

[travis_mccormack]

Wow, thanks for the info, I never knew that.

Anyone else have any useful info?

I can use all I can get.

Thanks again

[mark_drutz]

<p>While modern lens design has changed this somewhat the formula for determining the f-stop is</p>

<p>f-stop = focal length/diameter of lens opening</p>

<p>So if you increase the focal length you have to increase the diameter of the lens opening to keep the f-stop constant.</p>

<p>for example: f/4= 100/25 f/4= 200/50 When the focal length went from 100 to 200, the lens opening went from 25 to 50.</p>

<p>See why you need math?</p>