f/4 vs f/6.3 and slower: how much background blur do you really give up?...
p.1 #1f/4 vs f/6.3 and slower: how much background blur do you really give up? (interactive comparison)
With lighter, slower super-teles like Sony’s new 600mm f/6.3 GM getting a lot of attention, I kept wondering what we actually give up in background blur compared to f/4 glass. I shoot the EF 400 DO II and 500 f/4L II on an R5 Mark II, and I wanted something better than “feel.”
I’m not a programmer, so I worked with Claude, Anthropic’s AI assistant, to build a comparison based on the actual defocus math. I asked the questions and pushed it in the direction I wanted, and Claude did the math, made the images, and built the interactive version.
The interactive version on my blog lets you switch f-stops, bird distance and background distance yourself: [https://slowroadsouth.com/background-blur-for-a-600mm-lens/]
A few things stood out:
At the same focal length, f/4 gives blur circles 1.58× wider than f/6.3 and 2.75× wider than f/11. That ratio never changes.
How much it matters depends on the scene. With a far bird and a close background, everything looks busy at any f-stop. With a closer bird and a distant background, f/4 melts it to smooth color, while f/6.3 and slower still show texture.
The light cost: f/6.3 is about 1⅓ stops slower than f/4, roughly 2.5× the ISO for the same shutter speed.
This is a geometric simulation.
I’m not a programmer, so I worked with Claude, Anthropic’s AI assistant, to build a comparison based on the actual defocus math. I asked the questions and pushed it in the direction I wanted, and Claude did the math, made the images, and built the interactive version.
The interactive version on my blog lets you switch f-stops, bird distance and background distance yourself: [https://slowroadsouth.com/background-blur-for-a-600mm-lens/]
A few things stood out:
At the same focal length, f/4 gives blur circles 1.58× wider than f/6.3 and 2.75× wider than f/11. That ratio never changes.
How much it matters depends on the scene. With a far bird and a close background, everything looks busy at any f-stop. With a closer bird and a distant background, f/4 melts it to smooth color, while f/6.3 and slower still show texture.
The light cost: f/6.3 is about 1⅓ stops slower than f/4, roughly 2.5× the ISO for the same shutter speed.
This is a geometric simulation.
Nov 11, 2026 at 07:08 PM
p.1 #2f/4 vs f/6.3 and slower: how much background blur do you really give up? (interactive comparison)
There's an online DoF calculator that also shows a preview https://dofsimulator.net/en/
Many opt for a slower lens because of the weight & cost.
If you're not shooting canopy birds/wildlife that are found under the foliage I'd opt for the 600/800 f/6.3
A Canon equivalent would've been preferred.
Many opt for a slower lens because of the weight & cost.
If you're not shooting canopy birds/wildlife that are found under the foliage I'd opt for the 600/800 f/6.3
A Canon equivalent would've been preferred.
Nov 11, 2026 at 07:33 PM
p.1 #3f/4 vs f/6.3 and slower: how much background blur do you really give up? (interactive comparison)
dolina wrote:
There's an online DoF calculator that also shows a preview https://dofsimulator.net/en/
Many opt for a slower lens because of the weight & cost.
If you're not shooting canopy birds/wildlife that are found under the foliage I'd opt for the 600/800 f/6.3
A Canon equivalent would've been preferred.
Thanks, that DoF simulator is a great tool, and it’s good to see it lines up with what I found.
You’re right that weight and cost are the big reasons people go slower, and for a lot of shooting they’re the right call. Your canopy point is exactly where f/4 earns its keep for me: under foliage you get both the low light and a busy background close behind the bird, so you need the extra stop and a third and the bigger blur circles.
And I agree about Canon. If they made a light 600 f/6.3 in RF mount, I’d be tempted for open-country and shorebird work. Until then, my 400 DO II and 500 f/4 cover both, and they already fit in my carry-on.
There's an online DoF calculator that also shows a preview https://dofsimulator.net/en/
Many opt for a slower lens because of the weight & cost.
If you're not shooting canopy birds/wildlife that are found under the foliage I'd opt for the 600/800 f/6.3
A Canon equivalent would've been preferred.
Thanks, that DoF simulator is a great tool, and it’s good to see it lines up with what I found.
You’re right that weight and cost are the big reasons people go slower, and for a lot of shooting they’re the right call. Your canopy point is exactly where f/4 earns its keep for me: under foliage you get both the low light and a busy background close behind the bird, so you need the extra stop and a third and the bigger blur circles.
And I agree about Canon. If they made a light 600 f/6.3 in RF mount, I’d be tempted for open-country and shorebird work. Until then, my 400 DO II and 500 f/4 cover both, and they already fit in my carry-on.
Nov 11, 2026 at 07:43 PM
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p.1 #4f/4 vs f/6.3 and slower: how much background blur do you really give up? (interactive comparison)
You could take your 400 and/or 500/4 and shoot your most common subjects and typical camera/subject/background distance ratios at f/4 and f/6.3 for real world results comparison. 
But yes, the farther away the subject and the closer it is to the background, the less difference a faster lens will make with respect to subject-background separation. OTOH, BIF against a clear sky makes no difference. A slower lens would not result in a subjective image quality downgrade yet would be easier to hand hold to track the bird.
Based on the simulator, if your priority or preference is for the most background blur possible, the faster lens is advantageous mostly in the close-close and medium-medium scenarios. That's kind of the 'sweet spot' range it seems as you're probably aiming for getting as frame-filling an image of the bird as possible.
It's the same for me with sports. Action along the far side of the field is poorly separated from the busy background and is captured more or less for the story-telling value it might provide, such as a major play of the game, rather than for the aesthetic quality of the images. It's a reason I prefer working from end zones looking down the field, because that generally provides the greatest subject-background separation and looks the nicest.
But yes, the farther away the subject and the closer it is to the background, the less difference a faster lens will make with respect to subject-background separation. OTOH, BIF against a clear sky makes no difference. A slower lens would not result in a subjective image quality downgrade yet would be easier to hand hold to track the bird.
Based on the simulator, if your priority or preference is for the most background blur possible, the faster lens is advantageous mostly in the close-close and medium-medium scenarios. That's kind of the 'sweet spot' range it seems as you're probably aiming for getting as frame-filling an image of the bird as possible.
It's the same for me with sports. Action along the far side of the field is poorly separated from the busy background and is captured more or less for the story-telling value it might provide, such as a major play of the game, rather than for the aesthetic quality of the images. It's a reason I prefer working from end zones looking down the field, because that generally provides the greatest subject-background separation and looks the nicest.
Nov 11, 2026 at 08:22 PM
p.1 #5f/4 vs f/6.3 and slower: how much background blur do you really give up? (interactive comparison)
Shoot at f6.3 with your lenses in different scenarios and see the results, forget about AI. Shoot against busy backgrounds and you'll se the differences.
Nov 11, 2026 at 11:21 PM
p.1 #6f/4 vs f/6.3 and slower: how much background blur do you really give up? (interactive comparison)
Last thing I worry about when there are so many easy blur options in post-processing. Photoshop > select subject > invert selection > apply whatever kind of blur you want in any strength you want > deselect. Voila, done.
Nov 11, 2026 at 11:33 PM