Trying to understand the effects of focal length

[Low_Sky]

The photo below was taken with my Sony point-and-shoot on auto with the following settings: Focal length = 25mm, f/7.1, 1/500th, ISO-80. I know this isn't a good photo, but I'm getting my first DSLR soon, and I'm trying to get familiar with what would have made this a good photo. My question here is specifically is about the effects of focal length, but if you have other insights I'd love to hear them.

Given that this was taken with a 25mm focal length and all other factors being equal, would a 50mm focal length have made this bird twice as big and shown half as much background, or am I misunderstanding the math involved? Following the same logic, would a 200mm lens get eight times closer (that would have the Apache filling almost the whole frame)?

I like this photo because the subject, what's in the background, and where this photo was taken from trigger some emotions for me, and I just think Apaches are cool. I'm trying to learn what equipment and techniques would have made this into a good photo instead of just another point and shoot snapshot. Thanks!

 

[Devinhullphoto]

I love shooting planes and helicopters when they are close enough to get a tight crop. If I was there I probably would have gotten a lot tighter on the heli just because I love detail shots of stuff like that but from what you wrote it looks like once you get a dslr you will e able to take photos that are great.

What kind are you getting?

 

[Big Mike]

Focal length has a direct affect on the field of view (FOV) that the camera sees/records. A longer focal length will have a narrower FOV and will typically call that 'zooming in'. A shorter focal length will give you a wider view.

Another factor is the size of the recording medium (size of the sensor). The larger the sensor, the wider the FOV will be, for a given focal length.

So if we consider your point & shoot camera, it almost certainly has a rather small sensor...so a focal length of 25mm is going to result in a narrow FOV....and to get a wide FOV on your camera, the lens probably zooms to a focal length in the range of single digit mms.

So how do we compare one camera/lens to another? We use an equivalency standard....which is 35mm film (or full frame DSLR sensors). So based on the size of the sensor, we can come up with a FOV equivalence factor (often called the 'crop factor'....and incorrectly refereed to as a focal length equivalence).

I don't know (off hand) what the factor is for your current camera....but you could find out pretty easily. For example, a web site like Digital Photography Review doesn't necessarily list the actual focal length range for a camera...but it lists the equivalent (based on the standard of 35mm film). (you didn't tell us the model of your camera, so I couldn't look it up for you).

So now that you are considering a DSLR...you need to know that most of them have one of two sensor sizes...APS-C or 'Full Frame'. For Canon's APS-C sized sensors, the FOV equivalence factor is 1.6...for Nikon, it's 1.5. Full frame is 1:1 (no factor).

So to more directly get to your question. A 25mm lens on a typical (APS-C) DSLR camera, is a fairly wide angle view...not 'zoomed in' at all. And on a full frame camera, it's down right wide angle.
A 50mm would get you a narrower FOV...but to really get 'closer'...you would be looking at at-least 200-300mm for a focal length. Professional sports and wildlife shooters, for example, will often shoot with something in the 300-600mm range.

 

[KmH]

Check your Sony P&S lens specs. That 25 mm focal length may be an equivalent focal length that has been adjusted because of the camera's image sensor size.

Understanding Camera Lenses

 

[Low_Sky]

Big Mike,

Thanks for the explanation. I've learned a bit about crop factor, and have been keeping that in mind as I try to decide which lenses I would need for my interests. I did however, completely overlook that my point and shoot would have a tiny sensor. I looked up my camera (a Sony DSC-T90) on the website you linked. I found that it has an equivalent focal length of 35-140mm. Since my camera was zoomed all the way in (or close to it, there's no hard stop between optical and digital zoom so I can never get full "4.0x"), I guess we can assume this photo was taken at roughly a 140mm equivalent.

Now that we have established that, I'd like to restate my original question. Does doubling focal length - all other factors being equal (including the ones I was overlooking in my last post - half the field of view, or is the math not as simple as that? I have read a number of tutorials that talk about the effect of focal length on depth of field and background, and overall composition by keeping the size of the subject the same in the final product. I am curious about the effects on the size of the subject if the camera isn't moved.

 

[Low_Sky]

Check your Sony P&S lens specs. That 25 mm focal length may be an equivalent focal length that has been adjusted because of the camera's image sensor size.

Understanding Camera Lenses

KmH,

The link you posted answered my question perfectly. About 1/4 of the way down the page there is a "Required Focal Length Calculator" that allowed me to play with some numbers and get the answer to my question. Thanks!

 

[EDL]

Big Mike,

Thanks for the explanation. I've learned a bit about crop factor, and have been keeping that in mind as I try to decide which lenses I would need for my interests. I did however, completely overlook that my point and shoot would have a tiny sensor. I looked up my camera (a Sony DSC-T90) on the website you linked. I found that it has an equivalent focal length of 35-140mm. Since my camera was zoomed all the way in (or close to it, there's no hard stop between optical and digital zoom so I can never get full "4.0x"), I guess we can assume this photo was taken at roughly a 140mm equivalent.

Now that we have established that, I'd like to restate my original question. Does doubling focal length - all other factors being equal (including the ones I was overlooking in my last post - half the field of view, or is the math not as simple as that? I have read a number of tutorials that talk about the effect of focal length on depth of field and background, and overall composition by keeping the size of the subject the same in the final product. I am curious about the effects on the size of the subject if the camera isn't moved.

Doubling focal length halves the FOV width horizontally, and also vertical height, so total area covered is 1/4th the original (area = width X height).

 

[Low_Sky]

Big Mike,

Thanks for the explanation. I've learned a bit about crop factor, and have been keeping that in mind as I try to decide which lenses I would need for my interests. I did however, completely overlook that my point and shoot would have a tiny sensor. I looked up my camera (a Sony DSC-T90) on the website you linked. I found that it has an equivalent focal length of 35-140mm. Since my camera was zoomed all the way in (or close to it, there's no hard stop between optical and digital zoom so I can never get full "4.0x"), I guess we can assume this photo was taken at roughly a 140mm equivalent.

Now that we have established that, I'd like to restate my original question. Does doubling focal length - all other factors being equal (including the ones I was overlooking in my last post - half the field of view, or is the math not as simple as that? I have read a number of tutorials that talk about the effect of focal length on depth of field and background, and overall composition by keeping the size of the subject the same in the final product. I am curious about the effects on the size of the subject if the camera isn't moved.

Doubling focal length halves the FOV width horizontally, and also vertical height, so total area covered is 1/4th the original (area = width X height).

Very succinctly said. Thanks!