Pixel size, resolution, and field of view, explained simply
A few weeks ago I posted a one-page cheat sheet on machine vision basics. It did better than anything I have made, and the comments turned into a free education. Optical engineers and PhDs jumped in with the next layer: object-space pixel size, why magnification is not resolution, when to bin. I did not know all of it cold. So I went and learned it, and I built the follow-up. This is that follow-up, written the way I needed it explained.
- There are two pixel sizes. Object-space pixel size, what one pixel covers on the sample, is the one that matters.
- Resolution comes from the lens. Sampling comes from the camera. Match them at Nyquist.
- Bigger pixels collect more light and lift signal to noise, at the cost of resolution. Binning trades the same way.
- Field of view is sensor size over magnification. Magnification and field always pull against each other.
- Your sensor is a rectangle that has to fit the round image circle, or the corners go dark.
- A calculator below lets you plug in your own camera and objective and see if you are under, matched, or over sampled.
01There are two pixel sizes
The first thing that tripped me up was the phrase pixel size. There are two of them and they are easy to mix up. Sensor pixel size is physical. It is the pitch of one pixel on the chip, often 6.5 microns on an sCMOS camera, and you read it off the spec sheet. Object-space pixel size is what one pixel covers out on your sample. That second number is the one that matters.
You get it by dividing the sensor pixel by your total magnification. The trap is the coupler. A 0.63x C-mount adapter on a 60x objective is really 37.8x, not 60x, and if you forget it, every number after that is wrong. If you only remember one thing from this whole post, remember object-space pixel size.
02Resolution is the lens, sampling is the camera
Resolution and sampling are not the same thing, and that took me too long to see. Optical resolution is what the lens can actually resolve, set by numerical aperture and the wavelength of light. A 1.4 NA lens in green light resolves down to about 0.18 microns, and the camera cannot beat that. Camera resolution is just how finely you sample that image with pixels. Your system is limited by whichever one is worse.
The match point is Nyquist: make your object pixel about half the optical limit. Sample coarser than that and two close things blur into one. That is undersampling. Sample much finer and you only make bigger files for no new detail. That is oversampling.
Both have their place. Undersampling is fine for screening, counting, tracking, and low-light work where you want speed and reach more than fine detail. Oversampling earns its cost for single-molecule localization, super-resolution, and deconvolution. Most everyday research lives at matched.
03Bigger pixels see more light
A pixel is a bucket for light. A bigger pixel has a wider mouth, so at the same exposure it collects more photons. More photons means a stronger signal against the same noise, which is a higher signal to noise ratio. Faint structure that a small pixel would lose in the noise starts to show up.
The catch is resolution. Spread the same sensor over bigger pixels and you sample coarser. Sensitivity goes up, fine detail goes down. Binning is how you make bigger pixels on demand. It electronically groups neighboring pixels and sums their signal into one larger superpixel. Bin when you need sensitivity and can give up resolution, when you want more speed, or to shrink file sizes. On older CCD cameras binning also cut noise. On modern sCMOS it helps file size and speed more than noise.
04Field of view is the trade for magnification
Field of view is how much of the sample you see at once. It is the sensor size divided by the total magnification, so magnification and field of view pull against each other. Higher magnification gives you finer sampling and a smaller window. Lower magnification gives you a bigger window and coarser sampling. There is no setting that gives you both.
For screening or tiling a slide, a bigger field means fewer tiles, faster runs, and less data to store. For resolving fine structure, you accept the smaller field. Pick the side your experiment needs and stop fighting the tradeoff.
05The sensor and the mount
The last piece is the camera mount, and it surprised me. A microscope projects a round image of a fixed size, called the image circle. Your sensor is a rectangle that has to fit inside that circle. Make the sensor too big and the corners fall outside the circle and go dark. That is vignetting, and it is why most scientific cameras are square or 4 by 3 rather than wide.
A reducing coupler, like a 0.5x or 0.63x, shrinks the projected image to fit a larger sensor, and as a bonus it widens your field of view. So sensor size, aspect ratio, and the coupler are really one decision: how to fit a rectangle inside a circle.
Start with three numbers from your own rig: your camera’s pixel size, your objective magnification, and your coupler. No coupler? Leave it at one. The tool multiplies objective by coupler to get total magnification, then divides your pixel size by that to get your object pixel, which is what one pixel covers on the sample.
Now read the verdict. Green is matched, you are sampling about right. Orange is undersampled, your pixels are bigger than the detail, which is fine when you want speed or a wide field. Amber is oversampled, you are spending pixels and light you do not need. Change one number at a time and watch the object pixel and field of view move. That is the whole game: matching your camera to your optics.
Scale & Sampling Calculator
Plug in your camera and objective to estimate object-space pixel size, the optical limit, field of view, and whether you are undersampled, matched, or oversampled. Same assumptions as the sheet: widefield lateral resolution, total mag = objective × coupler.
Example: 6.5 µm pixels through a 60x / 1.4 lens give 0.108 µm object pixels and sit near the Nyquist match point.
- Match target: object pixel ≈ optical limit / 2.
- FOV: sensor width / total magnification.
- Coupler: a 0.63x C-mount is part of total mag. Leave it in or every number shifts.
The one-page version, free
Everything above, plus the light-path diagram, the under, matched, and over sampling guide, the sensor-in-the-image-circle picture, and a how-to-choose-your-camera workflow, on one page you can keep next to the scope. No email required.