How to work with aperture with a zoom lens?

Question

I am by no means a photography expert so bare with me. I have a pentax k1000 camera for my high school photography class. My camera has a zoom in lense. My questions is how does the fstop(I think that’s what the aperture is called) is affected by this zoom in lense because I have used a pentax k1000 camera without a zoom in lense just fine. I am not sure how my aperture will be affected by this zoom in feature. For example If I am zooming in on something from afar to take a photo do I need to then have a bigger f-stop or smaller if I want the subject to be clear and the background blurry. I know I do not that much information but can anyone help me.

Answer 1

Ahhhh, the Pentax K-1000 - a.k.a the student special. Great reliable camera. I see that you want to blur out the backgrounds. Using a telephoto lens is perfect for that kind of effect. Fortunately for you, you're using an SLR so you can see exactly what kind of depth of field you'll get.



If you've been using a 50mm prime lens, you will notice that the zoom lens does not allow in nearly as much light. Maybe it'll allow in 1/2 to 1/4 the amount of light that a 50mm can. Because of this, you have to be aware of how slow you can hand hold a camera at the various focal lengths that your zoom lens has. Often the slowest shutter speed that one can use to avoid camera shake is 1/focal length, but your mileage may vary.



Along with the lens needing faster shutter speeds to avoid camera shake when not using a tripod, you will also notice that you'll get more subject blur too. As the focal length increases it magnifies the subject causing subtle movement to be more pronounced.



As has been mentioned, your zoom lens probably has a maximum aperture that changes as the focal length increases. This happens because of the physical increase in the lens' length. As the light travels down through the lens it dims. In most cases, the light will dim by as much as 50% or 1 full stop. Your camera's light meter will detect this change and you'll see it in the viewfinder. As you zoom in, the needle in the viewfinder will move. You will then need to adjust your shutter speed to match the needle and the hoop.



I strongly suggest that you go to YouTube.com and watch some videos on depth of field and how to manipulate it. In short, the depth of field is affected by the focal length, focusing distance and aperture. Since focal length and focusing distance is chosen based on the subject, you will almost always use the aperture to increase or decrease your depth of field. If your zoom lens has a longer focal length, you will notice that you can get blurry backgrounds easier with it than say a wide-angle lens.



Take a few rolls and definitely keep a journal with you where you can write down your focal length, shutter speed, aperture and ISO settings.

Answer 2

The other answers are missing an important point. On most zoom lenses, the aperture CHANGES as you zoom. When at the widest focal length the aperture may be something like 2.8 or 3.5, but when you zoom the lens to a more telephoto focal length such as 150mm, the aperture may close all the way down to f5.6 or even smaller, (higher f-stop number).



So, if you have the aperture set to it's widest opening when at the wide angle length, it is NOT going to be at that same aperture when you zoom the lens. This can make it problematic to get shallow depth of field when you are zooming the lens. Also, you may get blurred shots because the shutter speed has to become MUCH slower at the telephoto lens length in order to maintain a proper exposure. You may need a tripod or need to use a higher ISO film to get sharp photos at the lens telephoto setting.



Now... there ARE zoom lenses that maintain a constant aperture throughout the zoom range... BUT ... they are large, heavy, and EXPENSIVE. I very, very much doubt such a lens is on that camera.

Answer 3

The concept does not change. Zoom in first and then adjust aperture size to what you see.



As to making backgrounds blurred, position subject as far away from the background as possible with the camera as near to the subject as possible with zoom to the max then set to lowest f/number available.

Answer 4

Im also no photography expert but i think your best bet would be laying around with the lense and seeing what works best for you. The camera and focal distance are all factors that effect the situation so try it out yourself and see what happens

Answer 5

Aperture works the same with a zoom lens as a fixed lens.



As you probably know, F-stops are the ratio of the aperture width to the focal length--for instance, if you're at f:8, the aperture is 1/8 the size of the focal length. This is why we use a colon, because it's a ratio.



So your question is: If you have a zoom lens and you set it at f:8, and then you zoom it and change the focal length, is it still at f:8. Actually that's a very good question! It's something most of us never think of.



In practice, it's not a problem. You can zoom a lens in and out without changing the f stop. I think it has to do with the way a zoom lens works, but I couldn't explain it further than to say you don't need to readjust the aperture after zooming, and it's not a problem.



EDIT



After re-reading the question, there's another thing. You know f-stop affects depth of field--smaller apertures give you more depth of field. Also longer focal lengths give you less depth of field. If you want to adjust depth of field you can zoom to your desired focal length and then adjust the aperture. Just understand that the same aperture will give you different depths at different zoom lengths.