Benefit of going below F/2.8?

Zoba132 said:

Maybe I need to accept that if I want a blurred background, I should focus on keeping the background far from my subjects, rather than relying solely on a low F-stop (which means I need to setup a scene for those).

Click to expand...

If you are comfortable with some basic mathematics, you can easily work out the amount of background blur that you will get in a particular situation.

The formula for background blur size is

blur size = entrance pupil diameter x (subject to background distance / camera to background distance)

where: entrance pupil diameter = focal length / F-number

The blur size given by this formula is measured on the same scale as your subject (i.e. whatever the camera is focused on).

An example: Suppose you use a 50mm lens at f/2.8. The entrance pupil is 50/2.8 = 18mm.

If the background is 20' from the camera and the subject is 5' from the camera, then the size of the background blur will be 18 x (15/20) = 13mm.

Imagine that your subject is holding a small coin of 13mm diameter; that will indicate the amount of the blurring of the background.

I hope this is helpful.

Thank you for the thoughtful post—it’s clear you’ve already put significant consideration into both the technical and practical aspects of your decision. You’ve articulated your needs well, and many of the trade-offs you’re weighing are common challenges faced by photographers working with fast-moving subjects in real-world environments.

Your primary objective—capturing sharp, well-exposed images of your children in mixed and often low-light conditions—places specific demands on your lens choice, particularly with regard to aperture, focal length, and handling. Let’s walk through the considerations point by point.

Aperture and Exposure Requirements

You’re absolutely right in identifying that your current kit lens (16–50mm f/3.5–5.6) quickly becomes limiting in low-light situations. At 35mm, being restricted to f/5.0 or higher significantly reduces the amount of available light, forcing higher ISO settings and/or slower shutter speeds—both of which can negatively impact image quality or sharpness when photographing active children.

Switching to a lens with a wider maximum aperture will allow you to maintain faster shutter speeds (e.g., 1/250–1/500) while keeping ISO at more manageable levels. This is particularly important for indoor settings or low-light environments where flash isn’t always practical or desirable. A maximum aperture of f/2.8 or wider is certainly a step in the right direction in terms of exposure flexibility.

Depth of Field and Subject Movement

Your concern about shallow depth of field is valid. While wide apertures like f/1.4 or f/1.7 are excellent for isolating subjects and achieving background separation, they also result in a much narrower plane of focus. This can be challenging with children who move unpredictably or are not aligned on the same focal plane. In those cases, even minor movements can cause one child to fall out of focus entirely.

This is a key reason many photographers working with multiple subjects—especially in dynamic situations—prefer to use apertures around f/2.8 or f/4. These settings offer a more forgiving depth of field while still providing some background blur, especially when there is sufficient distance between the subject and background.

Prime vs. Constant-Aperture Zoom

A fast prime such as a 35mm f/1.4 or f/2 would provide you with excellent low-light capability and more pronounced background separation. It can also produce a higher degree of subject isolation, which can be aesthetically pleasing, particularly for individual portraits or controlled compositions. However, primes are inherently less flexible in dynamic settings, especially if you’re trying to frame two subjects quickly or adjust your composition without moving.

On the other hand, a constant-aperture zoom such as the Sigma 18–50mm f/2.8 offers significantly more flexibility. You retain the ability to reframe rapidly—an advantage when working with moving children—and f/2.8 provides a two-stop improvement in light-gathering ability over your current kit lens at 35mm. While it won’t match the background blur of a faster prime, it still allows for reasonable subject separation, particularly when background distance is considered in your composition.

Your observation that a fast zoom “covers the same range” as your current lens is true in terms of focal length, but it differs meaningfully in terms of aperture and optical quality. The Sigma 18–50mm f/2.8 is an upgrade in both respects. For many photographers, this makes it a compelling alternative to a fast prime, particularly when the goal is to balance light-gathering ability with operational flexibility.

Background Separation in Practice

You are also correct in noting that aperture is just one factor in achieving background blur. Subject-to-background distance plays an equally critical role. At f/2.8, you can achieve noticeable background separation if your subject is positioned even a few feet in front of a background. While f/1.4 will produce more blur at closer distances, the practical usability of that aperture—especially with multiple subjects—is often limited.

If background blur is a key creative goal, you may find that composing with intentional distance between subject and background achieves the desired effect even at f/2.8. This can be a more reliable approach than trying to use extremely wide apertures in dynamic settings.

Conclusion

Given your use case—photographing active children in indoor and low-light settings—a fast zoom such as the Sigma 18–50mm f/2.8 offers a compelling combination of low-light capability, focal length flexibility, and manageable depth of field. It will be a noticeable upgrade over your current kit lens and will likely improve both your keeper rate and your ability to adapt to spontaneous moments.

That said, a fast 35mm prime remains a valuable tool, particularly if you find yourself wanting a more compact setup or are frequently shooting in very low light where maximum aperture is a priority. In the long run, many photographers end up with both: a high-quality standard zoom for everyday use and one or two fast primes for specific creative purposes.

Either path will serve you better than your current kit lens. The decision ultimately depends on whether you prioritize flexibility and reliability (the zoom) or light-gathering and subject isolation (the prime) in your current workflow.

Zoba132 said:

Maybe I need to accept that if I want a blurred background, I should focus on keeping the background far from my subjects, rather than relying solely on a low F-stop (which means I need to setup a scene for those).

Would love to hear you thoughts

Click to expand...

I think the f/2.8 zoom suggestion is a good one.

I don't think setting up scenes is what you want, but rather moving around looking for the most compelling perspective (which often means you are avoiding distracting backgrounds or otherwise getting them far enough away from the subject that even f/4 or narrower isn't an issue). This can mean moving up or down as well as from side to side (I spend a lot of time on my belly). Learn to previsualize what you can get with what you have rather than trying to force what you have to what you want.

I would add that it's helpful to have your subject close, but not too close. As an example, my girlfriend's dog playing catch, taken at f/4:



About all that's in-focus is her nose and eyes, which is exactly what I wanted.

A zoom lens is naturally more versatile than a prime, but you say you find yourself shooting at 35mm quite often.

An F/2.8 zoom coupled with noise reduction software can give you an extra five stops of shutter speed compared to your current set-up.

The extra two stops that an F/1.4 prime could give you, come with some caveats, aand more often than not require additional experience. However an F/1.4 prime can be shot at F/2.8 wheras an F/2.8 zoom cannot be shot at F/1.4.

That's why many people would recommend both a zoom and a prime, but even if you had both lenses and two cameras, you can only have one to your eye at any one moment, and that's where the final decision comes down to you knowing your environment, your desires, your equipment and your skills, and the balance between them. (And Lady Luck too, sometimes!)

Certainly, increasing the proportional distance between your subject and its background is an extremely effective (and free) way to induce separation with any lens.

Personally, when two people are almost in line, more often than not I find myself side-stepping to intentionally force one further out of focus. It's all individual choice, but researching the parameters is the correct way to inform that choice.

Good luck!

Zoba132 said:

Hello everyone,

In a previous thread, I was asking about a recommended 35mm prime lens for my Fuji (APS-C) camera. That discussion quickly turned into a rabbit hole (as photography conversations often do for beginners), so I decided to start a dedicated thread.

I'm currently using the Fuji kit lens (16-50mm). While it offers decent optical quality for its price, being a kit lens, it has a variable aperture (F/3.5 to F/5.6). At around 35mm, it’s already at F/5.

I mainly take pictures of my children (ages 3 and 5), and it’s nearly impossible to get them to pose properly or stand still for even a second. This forces me to use a shutter speed of around 1/250–1/500, which is challenging indoors (I have a Godox flash for that). Outdoors, when it gets darker or in restaurants with high/black walls, I often have to compensate with a high ISO (3200, and recently, I’ve been experimenting with 6400).

My original plan was to get a fast prime lens. I was debating between 35mm and 56mm. While I like the compressed look of portraits at 50mm with my current lens, it’s often difficult to step back far enough indoors. I enjoy half-body portraits, but 35mm is just more versatile overall (feel free to share your thoughts on this).

Click to expand...

I liked primes in range 23-30mm for family and kid shooting (on Sony apsc camera). I felt 35mm as kind of too tight, specially indoors. But of course depends on your favourite framing.

Zoba132 said:

So, a prime lens with a lower F-stop would let in more light, helping with my low-light issues. Another advantage of a prime lens is the improved background blur at wider apertures. Since I can't easily pose my children, a blurred background helps keep the focus on them when there’s a lot going on around them.

However, someone suggested that instead of getting a prime, I should replace my kit lens with the Sigma 18-50mm F/2.8,

Click to expand...

Also a good possibility, but I still count f2.8 on apsc as on the edge for interior shooting (without flash). ISO often at 6400 and above, background not quite sufficiently blurred.

Zoba132 said:

which maintains F/2.8 across the entire focal range. Their argument was that going below F/2.8 makes the depth of field too shallow.

Click to expand...

Not true in general. If you want to eg. shoot kid sitting on the floor, playing with toys, even f1.4 is usually OK for that. Heavily depends on framing and used focal length

Zoba132 said:

At first, I wasn’t keen on replacing my kit lens with something covering the same focal range—I wanted something new to play with. But their point stuck with me.

Often, I’m photographing both kids, and they’re rarely standing in a perfect line. Often one of their faces could easily end up out of focus. While I love the idea of an ultra-blurred background, I’m starting to doubt that I could consistently get sharp images of moving children at F/1.7 or even F/2.

Click to expand...

Yes, in this scenario is better to shoot f2.8/f4 in style spray and pray. I actually give up on such shooting, photos rarely end up as keepers.

Zoba132 said:

That made me rethink whether I really need a fast prime. Maybe F/2.8 is already pushing the limit, and going lower would only make photographing my kids harder.

Click to expand...

Depends on your style of shooting

Zoba132 said:

Also, I assume background blur will be more difficult to achieve at F/2.8. If I understand correctly, at F/1.7, I can get some blur if the background is about 1 meter away. At F/2.8, I’d need around 2 meters, and for strong blur, I’d probably need at least 3 meters at F/1.7—meaning even more distance at F/2.8. But maybe this isn’t a real issue, because getting a sharp photo at F/1.7 in the first place is already challenging.

Click to expand...

Not with current gear. Cameras have great AF and a lot of lenses are nice and sharp already at fastest aperture.

It's the opposite, closer you are to the subject, more blur will be introduced (with the same focal length)

Zoba132 said:

Maybe I need to accept that if I want a blurred background, I should focus on keeping the background far from my subjects, rather than relying solely on a low F-stop (which means I need to setup a scene for those).

Would love to hear you thoughts

Click to expand...

Zoba132 said:

Hello everyone,

In a previous thread, I was asking about a recommended 35mm prime lens for my Fuji (APS-C) camera. That discussion quickly turned into a rabbit hole (as photography conversations often do for beginners), so I decided to start a dedicated thread.

I'm currently using the Fuji kit lens (16-50mm). While it offers decent optical quality for its price, being a kit lens, it has a variable aperture (F/3.5 to F/5.6). At around 35mm, it’s already at F/5.

I mainly take pictures of my children (ages 3 and 5), and it’s nearly impossible to get them to pose properly or stand still for even a second. This forces me to use a shutter speed of around 1/250–1/500, which is challenging indoors (I have a Godox flash for that). Outdoors, when it gets darker or in restaurants with high/black walls, I often have to compensate with a high ISO (3200, and recently, I’ve been experimenting with 6400).

My original plan was to get a fast prime lens. I was debating between 35mm and 56mm. While I like the compressed look of portraits at 50mm with my current lens, it’s often difficult to step back far enough indoors. I enjoy half-body portraits, but 35mm is just more versatile overall (feel free to share your thoughts on this).

So, a prime lens with a lower F-stop would let in more light, helping with my low-light issues. Another advantage of a prime lens is the improved background blur at wider apertures. Since I can't easily pose my children, a blurred background helps keep the focus on them when there’s a lot going on around them.

However, someone suggested that instead of getting a prime, I should replace my kit lens with the Sigma 18-50mm F/2.8, which maintains F/2.8 across the entire focal range. Their argument was that going below F/2.8 makes the depth of field too shallow. At first, I wasn’t keen on replacing my kit lens with something covering the same focal range—I wanted something new to play with. But their point stuck with me.

Often, I’m photographing both kids, and they’re rarely standing in a perfect line. Often one of their faces could easily end up out of focus. While I love the idea of an ultra-blurred background, I’m starting to doubt that I could consistently get sharp images of moving children at F/1.7 or even F/2.

That made me rethink whether I really need a fast prime. Maybe F/2.8 is already pushing the limit, and going lower would only make photographing my kids harder.

Also, I assume background blur will be more difficult to achieve at F/2.8. If I understand correctly, at F/1.7, I can get some blur if the background is about 1 meter away. At F/2.8, I’d need around 2 meters, and for strong blur, I’d probably need at least 3 meters at F/1.7—meaning even more distance at F/2.8. But maybe this isn’t a real issue, because getting a sharp photo at F/1.7 in the first place is already challenging.

Maybe I need to accept that if I want a blurred background, I should focus on keeping the background far from my subjects, rather than relying solely on a low F-stop (which means I need to setup a scene for those).

Would love to hear you thoughts

Click to expand...

The only zoom lens I have for my Fuji system is the same as yours, the 16-50mm, I could not see any need for a better replacement unless it breaks, it gets the job done.

There are loads of budget friendly 16mm`s, 23mm`s and 27mm`s, 35mm as well but I find 35mm a little long working in tight spaces.

I generally preferer the slightly larger/heavier F1.4 lenses, they don`t cost a great deal more second hand.

A short while ago I picked up a 27mm Pancake on a whim, an impulse buy and was planning on sending it back and missed the return window but I later realised even at its widest aperture of f2.8 it is still useful, great for family days out with one body/one lens, a light weight compact package.


I later paired the 27mm with a LightPix Labs FlashQ Q20III, a tiny little DN20 flash paired with it own detachable transmitter, its just so useful for a bit of added fill and takes up very little room.


I might of said this before but generally speaking the wider you go in lens mm the greater the dof is at any given aperture (unless your focusing in really close, (you can still often achieve subject isolation with wider lenses, simply by getting in close)

Great analysis! I think the Sigma 18-50mm F/2.8 is a smart, flexible choice for photographing active kids. A consistent F/2.8, plus zoom versatility, can help you capture sharper moments without worrying about shallow depth issues. You can always add a fast prime later for creative portraits

Tom Axford said:

I hope this is helpful.

Click to expand...

It does! Always interesting to learn what's the math behind the basics things. Sometimes I'm so focused in the specific things- it's easy to forget all can be drill down to numbers at the end

Bob said:

Thank you for the thoughtful post—it’s clear you’ve already put significant consideration into both the technical and practical aspects of your decision. You’ve articulated your needs well, and many of the trade-offs you’re weighing are common challenges faced by photographers working with fast-moving subjects in real-world environments.

Click to expand...

Hello Bob! Sorry for the late response—something came up unexpectedly. Thank you for the detailed post; I really appreciate the time you put into every part of it! Loved every part of it!

Bob said:

Conclusion

Given your use case—photographing active children in indoor and low-light settings—a fast zoom such as the Sigma 18–50mm f/2.8 offers a compelling combination of low-light capability, focal length flexibility, and manageable depth of field. It will be a noticeable upgrade over your current kit lens and will likely improve both your keeper rate and your ability to adapt to spontaneous moments.

That said, a fast 35mm prime remains a valuable tool, particularly if you find yourself wanting a more compact setup or are frequently shooting in very low light where maximum aperture is a priority. In the long run, many photographers end up with both: a high-quality standard zoom for everyday use and one or two fast primes for specific creative purposes.

Either path will serve you better than your current kit lens. The decision ultimately depends on whether you prioritize flexibility and reliability (the zoom) or light-gathering and subject isolation (the prime) in your current workflow.

Click to expand...

Sounds like the Sigma 18-50mm is the right choice here. As a novice photographer, my current focus is on capturing moments with my family. Creative photography isn’t something I actively pursue (yet). The flexibility of a zoom lens, like the one you suggested, sounds exactly like what I need. I think it will ultimately help me figure out which prime lens to invest in—right now I tend to favor certain focal lengths, likely due to the limitations of my current aperture options. With f/2.8 available across the full range, I’ll be able to see where my real preferences lie—whether it's 24mm, 35mm, or 50mm.

Thanks again for the detailed answer!

OldNikonFan2025 said:

Zoba132 said:

Maybe I need to accept that if I want a blurred background, I should focus on keeping the background far from my subjects, rather than relying solely on a low F-stop (which means I need to setup a scene for those).

Would love to hear you thoughts

Click to expand...

I think the f/2.8 zoom suggestion is a good one.

Click to expand...

That's what I believe I will be going with!

OldNikonFan2025 said:

I don't think setting up scenes is what you want, but rather moving around looking for the most compelling perspective (which often means you are avoiding distracting backgrounds or otherwise getting them far enough away from the subject that even f/4 or narrower isn't an issue). This can mean moving up or down as well as from side to side (I spend a lot of time on my belly). Learn to previsualize what you can get with what you have rather than trying to force what you have to what you want.

Click to expand...

Yes. I think I often won't stop to take a look at the proper perspective. That's often because those smiling moments are so short with kids

OldNikonFan2025 said:

I would add that it's helpful to have your subject close, but not too close. As an example, my girlfriend's dog playing catch, taken at f/4:



About all that's in-focus is her nose and eyes, which is exactly what I wanted.

Click to expand...

Beautiful picture! Interesting it's 70mm. With 50mm I already feel like I have to take a big step backwards to frame thing correctly.

Aidan Williams said:

Aidan Williams wrote:.

Personally, when two people are almost in line, more often than not I find myself side-stepping to intentionally force one further out of focus. It's all individual choice, but researching the parameters is the correct way to inform that choice.

Click to expand...

"Interesting. I often see photos of animals taken from far away using a long focal length. Something that always bothered me is that while the face or nose is in focus, other parts—like a paw or hand—aren’t. I know this is often just a limitation of photography, but I still haven’t quite wrapped my head around the idea of intentionally having parts of a subject out of focus for artistic reasons."

Martin_99 said:

I liked primes in range 23-30mm for family and kid shooting (on Sony apsc camera). I felt 35mm as kind of too tight, specially indoors. But of course depends on your favourite framing.

Click to expand...

This makes me think again about 35mm. I really should try some 23mm shots.

Martin_99 said:

Yes, in this scenario is better to shoot f2.8/f4 in style spray and pray. I actually give up on such shooting, photos rarely end up as keepers.

Click to expand...

Can't help not trying. Some moments you really want to keep forever

Erickaa said:

Great analysis! I think the Sigma 18-50mm F/2.8 is a smart, flexible choice for photographing active kids. A consistent F/2.8, plus zoom versatility, can help you capture sharper moments without worrying about shallow depth issues. You can always add a fast prime later for creative portraits

Click to expand...

Thank you. That's I think what I will be going for!

The answer is already in your reply:

"but I still haven’t quite wrapped my head around the idea of intentionally having parts of a subject out of focus for artistic reasons."

You just need to decide what your 'subject' is before you press the shutter button.