Tag Archives: Photography

Why You need 1.4 lenses on Micro Four thirds

This post is NOT about underwater imaging. With the lockdown most of us have started using their cameras in the garden to shoot bugs, or birds or family members or abstracts.

In my instagram on the side you can see some examples of what I have been up to.

Shooting underwater is typically done at small apertures because of underwater optics issues. It is rare to shoot wide angle wider than f/5.6 on a MFT body or F/11 on full frame.

On land everything changes and you want to have as much light as possible coming into your camera to maximise dynamic range, bring out colours and minimised noise. Aperture controls not just how much light hits the sensor but also depth of field or I should say depth of focus.

Depth of field at equal level of magnification (size of the subject relative to the frame) depends only on the aperture of the lens. It does not matter if the lens is short or long once the subject fill your frame it is the f/number that influences depth of field.

2.8/2/1.4 is the Magic Number

Typically in full frame terms f/2.8 was a good lens, and the reason is quite simple if you shoot a classic 50mm lens from 1.5 meters away you will have 15 cm or half a foot depth of field. This is ideal to keep things in focus but also provide some background separation as objects blur as they move away from the area in focus. If you had a faster lens more light would go in the frame however you risk that nothing is in focus, for example nose and eye in focus and maybe ears not in focus.

And this is why 2.8 has been the magic number for full frame photography. If we move to an APSC sensor this becomes 2 and on MFT the magic number is 1.4. So 1.4 on a 25mm lens on MFT is equivalent to 2.8 on 50mm on full frame.

-20200211-13.jpg
Street Photography Night scene at 1.4

1.4 also gives plenty of light to your sensor so when you want to do some street photography or filming on MFT you can keep your ISO very low.

Exposure Value

Every scene has a level of illumination given in LUX and your camera needs to be able to expose for it with the right focus, with the required motion blur and lowest noise.

The scene in the image above is shot at f/1.4 1/60 ISO 640 let’s calculate the Ev taking into account the reference value is f/1 1 second and ISO 100.

1.4 means 1 stop 1/60 means 5.9 stops and 640 means 2.67 stops. So in total we have 6.9 stops of light taken away from aperture and shutter and 2.67 stops added by ISO gain. Total of 4.22 Ev using the formula Lux = 2.5 2^Ev we get 47 Lux which is the level of illumination of your living room in the evening with artificial lights.

If you had a slower lens like for example 2.8 to cover the same scene you needed to shoot at ISO 2500 this would have increased the noise, reduced the dynamic range and the colors.

2.8 Zooms are for outdoor

There are a number of great lenses for MFT cameras that are midrange zoom and have outstanding optical quality:

Panasonic 12-35
Olympus 12-40

The lenses above are constant aperture and weather sealed they are ideal for outdoor use however they do not offer a shallow depth of field for subject isolation as they really are f/5.6 in full frame equivalent and they are also slow meaning they will take you to the ISO 2500 zone if you try street photography or shooting movies in your living room.

Prime Rules

If you want fast lenses in MFT you need to have prime lenses, this is due to the physical constraint of the format.

Here my selection, I am not a fan of vintage lenses or full manual lenses, I like the best optical quality and if I want to add a vintage feel I do it in post.

From left: Panasonic 12mm, Sigma 16mm, Panasonic 25mm, Panasonic 42.5 all 1.4 lenses

In more detail:

Panasonic 12mm 1.4

The Panasonic 12mm 1.4 is an expensive lens that I use for astrophotography and gimbals plus low light narrow room indoor shots.

It is weather sealed, extremely sharp and fast to focus and works in full auto focus on a gimbal.

Home Sweet Home
Star Trail with 12mm 1.4
Gimbal
Sigma 16mm 1.4

The Sigma 16mm 1.4 must be the best value prime on the market for MFT lenses. I use it in street photos and for videos. It is almost a 35mm full frame lens.

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street photography with Sigma 16mm
Garden Overview
Panasonic 25mm

The Panasonic 25mm is a workhorse for small group portraits and ideal lens for movie style video.

25mm 1.4
Kids video with 25mm
Nocticron 42.5

The Panasonic 42.5 Nocticron is probably the best portrait lens on MFT and one of the best lenses overall.

Nocticron portrait

Why not Olympus/Others?

Of course there are equivalent primes from other brands for all focal lengths except the 12mm. They will perform equally and as long as they can go to 1.4 all is good. I use Panasonic bodies so tend to have Panasonic lenses and I buy Sigma since a long time but this is personal. There are tons of reviews on which lenses to choose etc etc but is not my place to do such comparisons.

How about Video?

Even more essential to have fast primes for video as you are constrained in the shutter speed you can use.

Using a 1.4 lens at 1/50 you can shoot several scenes at different ISO

ISOLuxTypical Scene
200125Dark day
40063Indoors low lit areas
80032full overcast sunset/sunrise very dark indoor
160015Near twilight
32008After Twilight dark
64004dark
128002very dark
256001Candlelight
Aperture vs environment

For my purposes this adequate for reference underwater scenes at 3.5 means I can cover 100 Lux in ambient light in movie mode before turning on the lights.

Conclusion

If you find yourselves with grainy images or videos invest in fast lenses. A lens is the eye of your camera and the sensor is the brain. Think about getting better lenses before investing in a new camera and consider that if you need to go in lower light it is not always true that getting a bigger sensor will help considering the limitation of depth of field so you may want to think about lights.

SNR in Digital Cameras in 2020

There are significant number of misconceptions about noise in digital cameras and how this depends on variables like the sensor size or the pixel size. In this short post I will try to explain in clear terms the relationship between Signal Noise Ratio (SNR) and sensor size.

Signal (S) is the number of photons captured by the lens and arriving on the sensor, this will be converted in electric signal by the sensor and digitised later on by an Analog Digital Converter (ADC) and further processed by Digital Signal Processors (DSP). Signal depending on light is not affected by pixel size but by sensor size. There are many readings on this subject and you can google it yourself using sentences like ‘does pixel size matter’. Look out for scientific evidence backed up by data and formulas and not YouTube videos.

S = P * e where P is the photon arrival rate that is directly proportional to the surface area of the sensor, through physical aperture of the lens and solid angle of view, and e is the exposure time.

This equation also means that once we equalise lens aperture there is no difference in performance between sensors. Example two lenses with equivalent field of view 24mm and 12mm on full frame and MFT with crop 2x when the lens aperture is equalised produce the same SNR. Considering a full frame at f/2.8 and the MFT at f/1.4 gives the same result as 24/2.8=12/1.4 this is called constrained depth of field. And until there is sufficient light ensures SNR is identical between formats.

Noise is made of three components:

  1. Photon Noise (PN) is the inherent noise in the light, that is made of particles even though is approximated in optics with linear beams
  2. Read Noise (RN) is the combined read noise of the sensor and the downstream electronic noise
  3. Dark Current Noise (DN) is the thermal noise generated by long exposure heating up the sensor

I have discovered wordpress has no equation editor so forgive if the formulas appear rough.

Photo Noise is well mapped by Poisson distribution and the average level can be approximated with SQRT(S).

The ‘apparent’ read noise is generally constant and does not depend on the signal intensity.

While 3 is fundamental to Astrophotography it can be neglected for majority of photographic applications as long as the sensor does not heat up so we will ignore it for this discussion.

If we write down the Noise equation we obtain the following:

Noise=sqrt({PN}^2+{RN}^2+{DN}^2)

Ignoring DN in our application we have two scenarios, the first one is where the signal is strong enough that the Read Noise is considerably smaller than Photon Noise. This is the typical scenario in standard working conditions of a camera. If PN >> RN the signal to noise ratio becomes:

SNR =sqrt S

S is unrelated to pixel size but is affected by sensor size. If we take a camera with a full frame and one with a 2x crop factor at high signal rate the full frame camera and identical f/number it has double the SNR of the smaller 2x crop. Because the signal is high enough this benefit is almost not visible in normal conditions. If we operate at constrained depth of field the larger sensor camera has no benefit on the smaller sensor.

When the number of photons collected drops the Read Noise becomes more important than the photon noise. The trigger point will change depending on the size of the sensor and smaller sensor will become subject to Read Noise sooner than larger sensors but broadly the SNR benefit will remain double. If we look at DxOMark measurements of the Panasonic S1 full frame vs the GH5 micro four thirds we see that the benefit is around 6 dB at the same ISO value, so almost spot on with the theory.

Full Frame vs MFT SNR graph shows 2 stop benefit over 2x crop

Due to the way the curve of SNR drops the larger sensor camera will have a benefit or two stops also on ISO and this is the reason why DxOMark Sport Score for the GH5 is 807 while the S1 has a sport score of 3333 a total difference of 2.046 stops. The values of 807 and 3333 are measured and correspond to 1250 and 5000 on the actual GH5 and S1 cameras.

If we consider two Nikon camera the D850 full frame and the D7500 APSC we should find the difference to be one stop ISO and the SNR to drop at the same 3 dB per ISO increment.

The graphic from DxoMark confirms the theory.

Full Frame vs APSC SNR graph shows 1 stop benefit over 1.5x crop

If the SNR does not depend on pixel size, why do professional video cameras and, some high end SLR, have smaller pixel count? This is due to a feature called dual native ISO. It is obvious that a sensor has only one sensitivity and this cannot change, so what is happening then? We have seen that when signal drops, the SNR becomes dominated by the Read Noise of the sensor so what manufacturers do is to cap the full well capacity of the sensor and therefore cap the maximum dynamic range and apply a much stronger amplification through a low signal amplifier stage. In order to have enough signal to be effective the cameras have large pixel pitch so that the maximum signal per pixel is sufficiently high that even clipped is high enough to benefit from the amplification. This has the effect of pushing the SNR up two stops on average. Graphic of the read noise of the GH5s and S1 show a similar pattern.

Panasonic Dual Gain Amplifier in MFT and Full Frame cameras shows knees in the read noise graphs

Sone manufacturers like Sony appear to use dual gain systematically even with smaller pixel pitch in those cases the benefit is reduced from 2 stops to sometimes 1 or less. Look carefully for the read noise charts on sites like photonsforphotos to understand the kind of circuit in your camera and make the most of the SNR.

Because most of the low light situation have limited dynamic range, and the viewer is more sensitive to noise than DR, when the noise goes above a certain floor the limitation of the DR is seen as acceptable. The actual DR is falling well below values that would be considered acceptable for photography, but with photos you can intervene on noise in post processing but not DR, so highest DR is always the priority. This does not mean however that one should artificially inflate requirements introducing incorrect concepts like Useable DR especially when the dual gain circuit reduce maximum DR. Many cameras from Sony and Panasonic and other manufacturers have a dual gain amplifier, sometimes advertised other times not. A SNR of 1 or 0 dB is the standard to define useable signal because you can still see an image when noise and signal are comparable.

It is important to understand that once depth of field is equalised all performance indicators flatten and the benefit of one format on the other is at the edges of the ISO range, at very low ISO values and very high ISO and in both cases is the ability of the sensor to collect more photons that makes the difference, net of other structural issues in the camera.

As majority of users do not work at the boundaries of the ISO range or in low light and the differences in the more usual values get equalised, we can understand why many users prefer smaller sensor formats, that make not just the camera bodies smaller, but also the lenses.

In conclusion a larger sensor will always be superior to a smaller sensor camera regardless all additional improvement made by dual gain circuits. A full frame camera will be able to offer sustained dynamic range together with acceptable SNR value until higher ISO levels. Looking for example at the Panasonic video orientated S1H the trade off point of ISO 4000 is sufficient on a full frame camera to cover most real-life situation while the 2500 of the GH5s leaves out a large chunk of night scenes where in addition to good SNR, some dynamic range may still be required.