Does sensor size matter when I'll be cropping the image anyway?

Question

Background

I like taking photos of formula 1 cars. I try and sit close to corners where they're going as slow as possible; this means sitting about 100m/330ft away from cars going 100kph/60mph. I'm currently using a point and click Samsung that's about 10 years old. Not got it on me to check model no, but the specs are:

• 1/2.3" sensor
• 10MP
• Max 31.5mm focal length

I always use the maximum focal length to get maximum magnification. This normally leaves the car as quite a small part of the picture, so I digitally crop it afterwards.

The picture quality is OK, but I want better. I am therefore looking at buying a new camera, and am trying to work out what specs will give me a better quality image. I'm mainly considering compact superzoom cameras, for size and weight reasons. For these cameras the lens isn't changeable, and with the current options on the market a bigger sensor comes with the trade-off of a shorter focal length.

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Focal length vs sensor

I've read mattdm's excellent answer on field of view. My understanding based on that is that the focal length is what determines the ratio between the real size of an object and how big it appears on the sensor. With the same focal length a bigger sensor will give you a wider field of view, meaning you'll see more background. But if I'm going to crop it to the f1 car anyway, this is useless to me.

So, presumably, all I need is to find the camera with the longest focal length, the sensor size doesn't matter. And that's true focal length, not 35mm equivalent.

Question: Am I missing something? Bigger sensors are more expensive, so I expect they should be better. But maybe they're not actually better for the specific type of photo I'm taking. Maybe I'm failing to take account of other important aspects of a bigger sensor (better performance in low light, more pixels per square inch) which would make the quality of the cropped photo better for the same focal length. Maybe it's actually 35mm equivalent focal length I should compare and not actual focal length.

Answer 1

My understanding based on that is that the focal length is what determines the ratio between the real size of an object and how big it appears on the sensor.

You're correct, and this actually answers your question. If you're going to use a particular focal length, and you want a distant object fill as much of the image as possible, then it follows that:

• Yes! Sensor size does matter.

• Assuming the sensors have the same pixel dimensions, your needs will be better served by a smaller sensor.

I'm sure Matt's answer explains the relationship between sensor size and angle of view. If you consider that, and if you're comparing two sensors with similar pixel dimensions but different physical size, then the larger one will use those pixels to cover a larger area, while the smaller one will pack the pixels more tightly. That means the smaller one will give you a higher resolution image of the area you want. You won't have to crop out the part you don't want, and more of the sensor's pixels will be used for the part that you do want.

Regarding your comment, yes, the assumption that the two sensors have similar pixel dimensions means that you get more pixels per unit area with the smaller sensor. If you're not comparing sensors of similar pixel dimensions, then it's still true that pixels per unit area is the factor that you should be looking at.

Also remember that more pixels/area will give you a higher resolution image in terms of pixels, but it doesn't necessarily mean better image quality. You also need a lens that's sharp enough to take advantage of those pixels. And for your purposes (shooting fast-moving objects), you need a lens that's fast enough to avoid motion blur.

Finally, consider that a camera with interchangeable lenses (like a DSLR) gives you more choices with respect to focal length. The assumption that more pixels/area is better (for magnification) only holds up if you're using the same focal length with each sensor. A larger sensor isn't necessarily a liability if you have the option of using a longer lens, but longer lenses do tend to be larger, heavier, and more expensive.

Bigger sensors are more expensive, so I expect they should be better. But maybe they're not actually better for the specific type of photo I'm taking.

Bigger sensors are more expensive in part because they're bigger -- you can't pack nearly as many of them onto a single silicon wafer. They do have advantages: bigger pixels spread out over a larger area means that they gather more light and see a wider field of view. Those traits don't help much if you're shooting distant objects in bright daylight, but they're great if you're shooting up close and/or in dim light. So yes, a larger sensor may not be the right choice for your application.

Answer 2

Caleb's answer is true; that a smaller senior will allow you to fill more of it from a greater distance, because it will change the apparent focal length compared to a 35mm equivalent.

However, after that you still have some choices to make.

Using a shorter focal length will in effect keep the car in frame for longer, allowing you to later crop out the best part.
The upside is that it makes the photograph easier to get. The downside is that you are throwing away a lot of potential detail.

Alternatively, using a longer lens will enable you to capture the car in its full glory, filling the frame.
The downside is that following a racing car on a long lens is a practised art - you will miss more than you find to start with, & a lot of those may be blurred. The upside is that the track looks great if it's motion-blurred behind the car, with the car sharp & standing out against it.

Search "formula one photography" on Google Images & you'll quickly spot the differences between
'news' photography - F8 & be there where the image is sharp right through &
'art' photography - a detail against a soft background.

You may need to choose your lens depending on which you would aim for.

Answer 3

As to composition: Any size sensor paired with an appropriate lens will deliver the view you seek. The real issue ; what are your hopes and ambitions. If your image is a real winner will it look great as the centerfold in Sports Illustrated? Bigger is better! The camera is just a tool. We size them based partially on convenience. Small cameras fit in the pocket, large cameras are heavy and cumbersome. On the other hand, technology marches on. Today’s sub-miniature exceeds yesterday’s larger formats. My advice is “Hope springs eternal” your next image might win the Pulitzer. If this is your passion choose the best and the largest camera you can afford with an eye on ergonomics.