If your labels are indeed (batches of) images of shape [3, 3, 256, 256], then you have to figure out how they are “encoded” to give binary class labels. Could they be pure black-and-white images that happen to be encoded as three-channel RGB images?

This is a cool concept. It also makes it easy to visualize why an aperture of f/4 would be larger than an aperture of f/16. Physically, at f/4, your aperture blades are open much wider, as shown below:

I'm Spencer Cox, a landscape photographer based in Colorado. I started writing for Photography Life a decade ago, and now I run the website in collaboration with Nasim. I've used nearly every digital camera system under the sun, but for my personal work, I love the slow-paced nature of large format film. You can see more at my personal website and my not-exactly-active Instagram page.

You might have seen this in your camera before. On your camera’s LCD screen or viewfinder, the f-stop looks like this: f/2.8, f/4, f/5.6, f/8, f/11, and so on. Sometimes, it will be shown without a slash in between like f2.8, or with a capital “F” letter in the front like F2.8, which means the exact same thing as f/2.8. These are just examples of different f-stops, and you might come across much smaller numbers like f/1.2 or much larger ones like f/64.

The images are of size torch.Size([3, 3, 256, 256]) and not torch.Size([3, 256, 256]). The first 3 is the batch size, the second one is for the 3 RGB channels, and the 256s are the image dimensions.

Luckily, you have the building blocks. Aperture and f-stop aren’t complicated topics, but they can seem a bit counterintuitive for photographers who are just starting out. Hopefully, this article clarified some of the confusion, and you now have a better understanding of the fundamentals of aperture.

Typically, the “maximum” aperture of a lens, which is also often referred to as “wide-open” aperture, will be something like f/1.4, f/1.8, f/2, f/2.8, f/3.5, f/4, or f/5.6.

I’m new to machine learning and PyTorch, and I’m stuck on this error which seems really simple but I can’t find where to fix it:

So, yes, get rid of the final x = x.view(x.size(0), -1) (and the commented-out x = self.linear(x)), and have your model output the result of the final Conv2d (self.end) layer. And, yes, out = 1 is correct.

A lot of photographers ask me an interesting question: What does the “f” stand for in f-stop, or in the name of aperture (like f/8)?

I was calling w_pred = net(V.float) instead of w_pred = net(V.float()). It’s written right there in the error that I kept looking at but I kept missing it until I posted it here

F-stop vsaperture

That’s a great question. I constantly use my cameras in colder weather than 32F, no matter what they’re rated for. Even sub-zero temperatures (still talking Fahrenheit) don’t concern me, although I don’t think the camera warranty would still apply if something did go wrong in those temperatures.

For example, say that you have an 80-200mm f/2.8 lens fully zoomed out to 80mm. If your f-stop is set to f/4, the diameter of the aperture blades in your lens will look exactly 20 millimeters across (80mm / 4), whereas at f/16, the diameter will be reduced to mere 5 millimeters (80mm / 16).

These blades form a small hole, almost circular in shape — your aperture. They also can open and close, changing the size of the aperture.

Thanks for the explanation. As a newbie I could not figure out why the aperture number increased in value even though the opening decreased. Knowing now that the value is actually a fraction has eliminated the confusion.

F-number calculator

This is very interesting! As you can see, in the f/4 photo, only a thin slice of the lizard’s head appears sharp. The background of the photo is very blurry. This is known as depth of field.

As a general rule (and as a requirement for convolutional networks), pytorch networks work with batches of inputs (and outputs and labels). If you want to process a single image, you still need to package it as a batch with a batch size of one.

f-number formula

If someone tells you to use a large aperture, they’re recommending an f-stop like f/1.4, f/2, or f/2.8. If someone tells you to use a small aperture, they’re recommending an f-stop like f/8, f/11, or f/16.

Unfortunately, you can’t just set any f-stop value that you want. At some point, the aperture blades in your lens won’t be able to close any smaller, or they won’t be able to open any wider.

Based on the name TissueDataset, the name UNet, and the use of BCEWithLogitsLoss as your loss criterion, I assume that you are performing binary semantic segmentation. That is, you wish to classify each pixel in your input as being in either “class 0” (background or healthy tissue or whatever) or “class 1” (foreground or diseased tissue or whatever).

As you would expect, there are differences between photos taken with a large aperture versus photos taken with a small aperture. Aperture size has a direct impact on the brightness of a photograph, with larger apertures letting in more light into the camera compared to smaller ones. However, that isn’t the only thing that aperture affects.

That is an important concept! Often, you’ll hear other photographers talking about large versus small apertures. They will tell you to “stop down” (close) or “open up” (widen) the aperture blades for a particular photo.

I always find that it’s easiest to understand depth of field by looking at photos, such as the comparison below. In this case, I used a relatively large aperture of f/4 for the photo on the left, and an incredibly small aperture of f/32 for the photo on the right. The differences should be obvious:

Adjusting your aperture is one of the best tools you have to capture the right images. You can adjust it by entering your camera’s aperture-priority mode or manual mode, both of which give you free rein to pick whatever aperture you like. That is why I only ever shoot in aperture-priority or manual modes!

By that same logic, an aperture of f/2 is much larger than an aperture of f/16. If you ever read an article online that ignores this simple fact, you’ll be very confused.

These are the main aperture “stops,” but most cameras and lenses today let you set some values in between, such as f/1.8 or f/3.5.

Photographers generally don’t care as much about the smallest or “minimum” aperture that the lens allows, which is why manufacturers don’t put that information in the name of the lens. However, if it matters to you, you will always be able to find this specification on the manufacturer’s website. A lens’s smallest aperture is typically something like f/16, f/22, or f/32.

Thank you so much for explaining this topic in a way I finally understand ! I have just started photography using a Nikon d3200 with 70-300mm lens primarily for wildlife .. I wish I had read this article before I started !

Edit 2: Using sizes (3, 256, 256) for images and (1, 256, 256) for labels, and removing .astype(int) from the __getitem__ method gives this error:

On the flip side, you should be able to see why landscape photographers prefer using f-stops like f/8, f/11, or f/16. If you want your entire photo sharp out to the horizon, this is what you should use.

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You can think of an aperture of f/8 as the fraction 1/8 (one-eighth). An aperture of f/2 is equivalent to 1/2 (one-half). An aperture of f/16 is 1/16 (one-sixteenth). And so on.

Since people care so much about maximum aperture, camera manufacturers decided to include that number in the name of the lens. For example, one of my favorite lenses is the Nikon 20mm f/1.8G. The largest aperture it offers is f/1.8.

Take a look inside your camera lens. If you shine a light at the proper angle, you’ll see something that looks like this:

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If you have a 50mm f/1.4 lens, the largest aperture you can use is f/1.4. Professional constant aperture zoom lenses like a 24-70mm f/2.8 will have f/2.8 as their maximum aperture at every focal length. Whereas cheaper consumer-grade lenses such as 18-55mm f/3.5-5.6 will have their maximum aperture change depending on focal length. At 18mm, the maximum is at f/3.5, while at 55mm, it changes to f/5.6. In between is a gradual shift from one to the other.

f-stop photography

One of my friends once had the mirror of his DSLR break in extremely cold weather – not a problem with mirrorless cameras, of course.

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Clearly, the model returns a size of 65536, which is 256*256, converting the image to a line. But how do I convert it back to a square? And how do I get rid of the batch size’s 3?

The other more important impact is depth of field – the amount of your photo that appears to be sharp from front to back. For example, the two illustrations below have different depths of field, depending on the size of aperture:

In any event, you have to process your labels “images” to be single-channel binary labels (of type float). (Your labels don’t actually have to be pure binary, that is, exactly zero or one – they can be probabilistic labels that run from zero to one.)

Working f-number

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The second page of our aperture article dives into every single effect of aperture in your photos. It includes things like diffraction, sunstars, lens aberrations, and so on. However, as important as all that is, it’s not what you really need to know – especially at first.

Why is your aperture written like that? What does something like “f/8” even mean? Actually, this is one of the most important parts about aperture: it’s written as a fraction.

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If your labels are indeed (batches of) images of shape [3, 3, 256, 256], then you have to figure out how they are “encoded” to give binary class labels. Could they be pure black-and-white images that happen to be encoded as three-channel RGB images?

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Usually, the sharpest f-stop on a lens will occur somewhere in the middle of this range — f/4, f/5.6, or f/8. However, sharpness isn’t as important as things like depth of field, so don’t be afraid to set other values when you need them. There’s a reason why your lens has so many possible aperture settings.

VERY helpful article for this novice. As you are in Colorado, perhaps you can help on this topic: I’m going on a northern lights tour in a few weeks, and am considering getting a camera better than my iPhone. I’ve read many articles recommending various cameras (Sony a6000 keeps coming up), but then when looking at specs, most [affordable in my budget] cameras state an operating temperature only down to 32f. I’ll be in Norway with tens in the teens and 20’s. Are the manufacturer specs to be taken seriously for real-world use? Would I ruin a camera using it in temps 20 degrees cooler than its rating?

As we have previously defined, aperture is basically a hole in your camera’s lens that lets light pass through. It’s not a particularly complicated topic, but it helps to have a good mental concept of aperture blades in the first place.

As a beginner photographer, you might have heard of such terms as f-stop or f-number and wondered what they actually mean. In this article, we will dive into these in detail and talk about how to use them for your photography.

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Below are some examples of photographs captured at different f-stops from f/2.8 to f/16, to give you an idea of how they are used in the field:

You already know the answer to this question, because aperture is a fraction. Clearly, 1/8 is larger than 1/22. So, f/8 is the larger aperture.

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The input to your model should have shape [nBatch, nChannels, H, W], where in your case nChannels = 3 and presumably correspond to the RGB channels of a color image, and have type float (or double).

You can think of depth of field as a glass window pane that intersects with your subject. Any part of your photo that intersects with the window glass will be sharp. The thickness of the glass changes depending upon your aperture. At something like f/4, the glass is relatively thin. At something like f/32, the glass is very thick. Also, depth of field falls off gradually rather than dropping sharply, so the window glass analogy is definitely a simplification.

The f-stop, which is also known as the f-number, is the ratio of the lens focal length to the diameter of the entrance pupil. If you did not understand that, don’t worry, because there is a much easier explanation of it for beginners. In very simple language, f-stop is the number that your camera shows you when you change the size of the lens aperture.

Instead, just know that the two biggest reasons to adjust your aperture are to change brightness (exposure) and depth of field. Learn those first. They have the most obvious impact on your images, and you can always read about the more minor effects later.

Aperture f numbercanon

Of course, putting everything into practice is another matter. Even if this entire article makes sense for now, you’ll still need to take hundreds of photos in the field, if not thousands, before these concepts become completely intuitive.

Edit 1: Getting rid of the line x = x.view(x.size(0), -1) … Edit 2: Using sizes (3, 256, 256) for images and (1, 256, 256) for labels, and removing .astype(int) from the __getitem__ method

Hopefully, you know how fractions work. 1/2 cup of sugar is much more than 1/16 cup of sugar. A 1/4 pound burger is larger than a 1/10 pound slider.

F-number lens

Edit 1: Getting rid of the line x = x.view(x.size(0), -1) and using a batch size of 4 instead of 3 for clarity, the error becomes:

Quite simply, the “f” stands for “focal length”. When you substitute focal length into the fraction, you’re solving for the diameter of the aperture blades in your lens. (Or, more accurately, the diameter that the blades appear to be when you look through the front of the lens).

Why is large maximum aperture in a lens so important? Because a lens with a larger maximum aperture lets more light into the camera. For example, a lens with a maximum aperture of f/2.8 lets in twice as much light when compared to a lens with a maximum aperture of f/4.0. This difference could be a big deal when shooting in low-light conditions.

Note, UNet does not (typically) have H and W wired into it – the same UNet can be trained on, and perform inference on images of differing shapes – but any given batch has to consist of images of the same shape.

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Aperture f numberphotography

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Thank you so much for your detailed answer! I am indeed trying to solve a segmentation problem. Here is an example of an image and its label:

If I open a label image on Photoshop, I can see the image mode is set to Grayscale and not RGB, so the shape should be [3, 1, 256, 256], right?

A lot of photographers really care about the maximum aperture that their lenses offer. Sometimes, they’ll pay hundreds of extra dollars just to buy a lens with a maximum aperture of f/2.8 rather than f/4, or f/1.4 rather than f/1.8.

This is why portrait photographers love f-stops like f/1.4, f/2, or f/2.8. They give you a pleasant “shallow focus” effect, where only a thin slice of your subject is sharp (such as your subject’s eyes). You can see how that looks here: