zoom verses aperture

[Pudda]

(Nikon D7000...18-105 lensy thingy on the front of the camera) regardless of what mode you are in (auto or manual) when you zoom in or out the fstop read out changes.... i.e. zoom out - fstop gets lower. Now, I understand that the lower the fstop, the bigger the aperture size, more light gets in but the depth of field is shortened..... but why does zooming (in/out) effect your aperture read out /display reading?? I though by setting the fstop, you SET the amount of light you want to get to the sensor... so why does it change when adjusting zoom.....??? Zoom is just framing your picture at the end of the day... Does that then effect your depth of field??.... or is that a dumb question..?

[kiwi]

It's not a dumb question, the zoom you have is called variable aperture. It's a lot more expensive for manufacturers to make zooms a fixed aperture through the zoom focal range, I'm afraid I'm not sure why from an engineering point of view

[ricktas]

Basically your lens will say something like 18-105 F3.5-f4.5

What that means is that at 18mm your maximum aperture is f3.5 and at 105 the maximum is f4.5. This differs from a lot of Pro level lenses where it will say 70-200 f2.8 for example, which means you can use f2.8 at any zoom length

[fillum]

The amount of light hitting the camera's sensor depends on the focal length of the lens as well as the aperture. When you extend the lens towards the 105mm end the amount of light reaching the sensor gets less with this lens, so the effective aperture will show a larger f-number.

I think the 18-105 has a variable maximum aperture of f/3.5 - f/5.6. The aperture will only change if you set it below f/5.6 at the wide angle end and then extend the zoom. It should only change to f/5.6, nothing higher. If originally set at f/5.6 or above it shouldn't change when changing the zoom length.

The depth of field will change, but it would change anyway when you change the zoom even if the aperture remains the same because depth of field also depends on the focal length.

Not sure how well I've explained this - let us know if you still have questions.


Cheers.

[Speedway]

If you are shooting in apture priority with the apture set to higher than the maximum the apture will not change but the shutter speed will to compensate for the light, if shooting in auto or shutter prioity the apture will change for the same reason.
Keith.

[James T]

... It's a lot more expensive for manufacturers to make zooms a fixed aperture through the zoom focal range, I'm afraid I'm not sure why from an engineering point of view

You need a bigger hole and therefore a bigger lump of glass to cover it, the long the focal length becomes. Just have a look at the front of your 400 f/2.8 and see how many 50 f/1.8 lenses you could fit in there. :) And thats with a faster aperture on the shorter lens.



...The depth of field will change, but it would change anyway when you change the zoom even if the aperture remains the same because depth of field also depends on the focal length...

It would in this case. But to be strictly correct, focal length per sé doesn't have an effect on depth of field - which is decided solely by aperture and magnification. That's been done to death in other threads though. :)

[OzzieTraveller]

G'day Pudda

you ask ...

..... but why does zooming (in/out) effect your aperture read out /display reading??

Okay - what you are seeing is quite normal and correct
What you are asking is "why is it so" ... it has nothing to do with the mode [shutter vs aperture] that the camera is being used.

Perhaps this sketch will help
http://i54.tinypic.com/9syvdk.jpg

in other words, every zoom lens in the world will change its "working aperture" when it is zoomed ... and in your camera via the settings shown on the screen, you are seeing this change take place

This is why every zoom lens is sold showing eg: "28-105mm, f2,8 ~ f4" as its description - the maker knows that as it zooms the working aperture changes from f2,8 thru f3,2 to f3,5 to f3,8 and finally to f4. You will see this extending effect - known as "lens-tromboning" as you zoom the lens from minimum mm's to maximum mm's

ps: there are some very expensive professional lenses that are specially made to overcome this - they are said to be 'constant' working aperture lenses ... these "professional" lenses will cost you maybe 5x more than "commercial" lenses

Hope this helps a bit
Regards, Phil